diff --git a/source/lib/sysdep/win/waio.cpp b/source/lib/sysdep/win/waio.cpp
index c93d0530d7..49ceeceee6 100755
--- a/source/lib/sysdep/win/waio.cpp
+++ b/source/lib/sysdep/win/waio.cpp
@@ -79,7 +79,6 @@ static void aio_h_cleanup()
 
 static bool is_valid_file_handle(const HANDLE h)
 {
-	SetLastError(0);
 	bool valid = (GetFileSize(h, 0) != INVALID_FILE_SIZE);
 	assert(valid);
 	return valid;
@@ -380,7 +379,9 @@ int aio_reopen(int fd, const char* fn, int oflag, ...)
 
 	// open file
 	DWORD flags = FILE_FLAG_OVERLAPPED|FILE_FLAG_NO_BUFFERING|FILE_FLAG_SEQUENTIAL_SCAN;
+	WIN_SAVE_LAST_ERROR;	// CreateFile
 	HANDLE h = CreateFile(fn, access, share, 0, create, flags, 0);
+	WIN_RESTORE_LAST_ERROR;
 	if(h == INVALID_HANDLE_VALUE)
 	{
 fail:
@@ -411,7 +412,6 @@ int aio_close(int fd)
 		return -1;
 	}
 
-	SetLastError(0);
 	if(!CloseHandle(h))
 		assert(0);
 	aio_h_set(fd, INVALID_HANDLE_VALUE);
@@ -437,6 +437,8 @@ static int aio_rw(struct aiocb* cb)
 debug_out("aio_rw cb=%p\n", cb);
 #endif
 
+	WIN_SAVE_LAST_ERROR;
+
 	const bool is_write = cb->aio_lio_opcode == LIO_WRITE;
 
 	if(!cb)
@@ -481,9 +483,7 @@ debug_out("aio_rw cb=%p\n", cb);
 	unsigned long opt = 0;
 	socklen_t optlen = sizeof(opt);
 	int sock = (int)(intptr_t)h;
-	DWORD last_err = GetLastError();
 	bool is_sock = getsockopt(sock, SOL_SOCKET, SO_TYPE, &opt, &optlen) != -1;
-	SetLastError(last_err);
 
 	// socket: no alignment calculation necessary
 	if(is_sock)
@@ -537,23 +537,26 @@ debug_out("aio_rw cb=%p\n", cb);
 	// this assumes little endian, but we're windows-specific here anyway.
 	*(size_t*)&r->ovl.Offset = ofs;
 
-
-assert(cb->aio_buf != 0);
-
-	SetLastError(0);
-
+	assert(cb->aio_buf != 0);
 	DWORD size32 = (DWORD)(size & 0xffffffff);
-ResetEvent(r->ovl.hEvent);
-	BOOL ok = (cb->aio_lio_opcode == LIO_READ)?
-		ReadFile(h, buf, size32, 0, &r->ovl) : WriteFile(h, buf, size32, 0, &r->ovl);
 
+	ResetEvent(r->ovl.hEvent);
+
+	BOOL ok;
+	if(cb->aio_lio_opcode == LIO_READ)
+		ok =  ReadFile(h, buf, size32, 0, &r->ovl);
+	else
+		ok = WriteFile(h, buf, size32, 0, &r->ovl);
+
+	// "pending" isn't an error
 	if(GetLastError() == ERROR_IO_PENDING)
 	{
-		// clear annoying error
 		SetLastError(0);
 		ok = true;
 	}
 
+	WIN_RESTORE_LAST_ERROR;
+
 	return ok? 0 : -1;
 }
 
diff --git a/source/lib/sysdep/win/wcpu.cpp b/source/lib/sysdep/win/wcpu.cpp
index f48e6000d3..c13836860b 100755
--- a/source/lib/sysdep/win/wcpu.cpp
+++ b/source/lib/sysdep/win/wcpu.cpp
@@ -50,6 +50,8 @@ int on_each_cpu(void(*cb)())
 
 static void check_speedstep()
 {
+	WIN_SAVE_LAST_ERROR;
+
 	// CallNtPowerInformation
 	// (manual import because it's not supported on Win95)
 	NTSTATUS (WINAPI *pCNPI)(POWER_INFORMATION_LEVEL, PVOID, ULONG, PVOID, ULONG) = 0;
@@ -108,6 +110,8 @@ static void check_speedstep()
 		// ia32 code will get a second crack at it.
 		cpu_speedstep = (is_laptop)? 1 : 0;
 	}
+
+	WIN_RESTORE_LAST_ERROR;
 }
 
 
diff --git a/source/lib/sysdep/win/wdetect.cpp b/source/lib/sysdep/win/wdetect.cpp
index 2921f81026..9648784db8 100755
--- a/source/lib/sysdep/win/wdetect.cpp
+++ b/source/lib/sysdep/win/wdetect.cpp
@@ -176,6 +176,8 @@ int win_get_gfx_drv_ver()
 	CHECK_ERR(get_ogl_drv_name(ogl_drv_name, sizeof(ogl_drv_name)));
 
 
+	WIN_SAVE_LAST_ERROR;	// GetFileVersion*, Ver*
+
 	// don't want to return 0 on success - we'd need to duplicate free(buf).
 	// instead, set this variable and return that.
 	int ret = -1;
@@ -208,6 +210,8 @@ int win_get_gfx_drv_ver()
 	}
 	free(buf);
 
+	WIN_RESTORE_LAST_ERROR;
+
 	return ret;
 }
 
diff --git a/source/lib/sysdep/win/wfam.cpp b/source/lib/sysdep/win/wfam.cpp
index bbc95936d1..6ff79420c2 100755
--- a/source/lib/sysdep/win/wfam.cpp
+++ b/source/lib/sysdep/win/wfam.cpp
@@ -262,6 +262,10 @@ static int get_packet(FAMConnection* fc);
 
 int FAMMonitorDirectory(FAMConnection* const fc, const char* const _dir, FAMRequest* const fr, void* const user_data)
 {
+	int err = -1;
+	WIN_SAVE_LAST_ERROR;	// Create*
+
+	{
 	const std::string dir(_dir);
 
 	GET_APP_STATE(fc, state);
@@ -272,14 +276,14 @@ int FAMMonitorDirectory(FAMConnection* const fc, const char* const _dir, FAMRequ
 	// make sure dir is not already being watched
 	for(WatchIt it = watches.begin(); it != watches.end(); ++it)
 		if(dir == it->second->dir_name)
-			return -1;
+			goto fail;
 
 	// open handle to directory
 	const DWORD share = FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE;
 	const DWORD flags = FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OVERLAPPED;
 	const HANDLE hDir = CreateFile(_dir, FILE_LIST_DIRECTORY, share, 0, OPEN_EXISTING, flags, 0);
 	if(hDir == INVALID_HANDLE_VALUE)
-		return -1;
+		goto fail;
 
 	// assign a new (unique) request number. don't do this earlier - prevents
 	// DOS via wasting reqnums due to invalid directory parameters.
@@ -287,7 +291,8 @@ int FAMMonitorDirectory(FAMConnection* const fc, const char* const _dir, FAMRequ
 	if(last_reqnum == INT_MAX)
 	{
 		debug_warn("FAMMonitorDirectory: request numbers are no longer unique");
-		return -1;
+		CloseHandle(hDir);
+		goto fail;
 	}
 	const int reqnum = ++last_reqnum;
 	fr->reqnum = reqnum;
@@ -300,9 +305,8 @@ int FAMMonitorDirectory(FAMConnection* const fc, const char* const _dir, FAMRequ
 	hIOCP = CreateIoCompletionPort(hDir, hIOCP, key, 0);
 	if(hIOCP == 0 || hIOCP == INVALID_HANDLE_VALUE)
 	{
-fail:
 		CloseHandle(hDir);
-		return -1;
+		goto fail;
 	}
 
 	// create Watch and associate with FAM structs
@@ -324,8 +328,13 @@ fail:
 	// instead of only at the next call to FAMPending.
 	PostQueuedCompletionStatus(hIOCP, 0, key, 0);
 	get_packet(fc);
+	}
 
-	return 0;
+	err = 0;
+
+fail:
+	WIN_RESTORE_LAST_ERROR;
+	return err;
 }
 
 
diff --git a/source/lib/sysdep/win/win.cpp b/source/lib/sysdep/win/win.cpp
index b9363d1bda..37c156b174 100755
--- a/source/lib/sysdep/win/win.cpp
+++ b/source/lib/sysdep/win/win.cpp
@@ -28,6 +28,12 @@
 #include <malloc.h>
 
 
+void sle(int x)
+{
+	SetLastError((DWORD)x);
+}
+
+
 //
 // these override the portable stdio versions in sysdep.cpp
 // (they're more convenient)
diff --git a/source/lib/sysdep/win/win_internal.h b/source/lib/sysdep/win/win_internal.h
index 34fac415f5..96a785f5cf 100755
--- a/source/lib/sysdep/win/win_internal.h
+++ b/source/lib/sysdep/win/win_internal.h
@@ -316,4 +316,8 @@ extern void win_unlock(uint idx);
 #define WIN_REGISTER_FUNC(func) static int func(); static int(*p##func)(void) = func
 
 
+#define WIN_SAVE_LAST_ERROR DWORD last_err__ = GetLastError();
+#define WIN_RESTORE_LAST_ERROR STMT(if(last_err__ != 0 && GetLastError() == 0) SetLastError(last_err__););
+
+
 #endif	// #ifndef WIN_INTERNAL_H
diff --git a/source/lib/sysdep/win/wposix.cpp b/source/lib/sysdep/win/wposix.cpp
index 9d1964cd48..2dc72c20af 100755
--- a/source/lib/sysdep/win/wposix.cpp
+++ b/source/lib/sysdep/win/wposix.cpp
@@ -66,8 +66,13 @@ int open(const char* fn, int oflag, ...)
 		va_end(args);
 	}
 
+	WIN_SAVE_LAST_ERROR;
+
 	int fd = _open(fn, oflag, mode);
 
+	WIN_RESTORE_LAST_ERROR;
+
+
 	// open it for async I/O as well (_open defaults to deny_none sharing)
 	if(fd > 2)
 	{
@@ -200,9 +205,18 @@ struct dirent* readdir(DIR* dir)
 {
 	_DIR* d = (_DIR*)dir;
 
+	DWORD last_err = GetLastError();
+
 	if(d->not_first)
 		if(!FindNextFile(d->handle, &d->fd))
+		{
+			// don't pass on the "error"
+			if(GetLastError() == ERROR_NO_MORE_FILES)
+				SetLastError(last_err);
+			else
+				debug_warn("FindNextFile failed");
 			return 0;
+		}
 	d->not_first = true;
 
 	d->ent.d_ino = 0;
@@ -396,6 +410,8 @@ int pthread_mutex_destroy(pthread_mutex_t* m)
 
 void* mmap(void* start, size_t len, int prot, int flags, int fd, off_t offset)
 {
+	WIN_SAVE_LAST_ERROR;
+
 	if(!(flags & MAP_FIXED))
 		start = 0;
 
@@ -434,13 +450,16 @@ void* mmap(void* start, size_t len, int prot, int flags, int fd, off_t offset)
 	if(!ptr || (flags & MAP_FIXED && ptr != start))
 		return MAP_FAILED;
 
+	WIN_RESTORE_LAST_ERROR;
+
 	return ptr;
 }
 
 
 int munmap(void* start, size_t /* len */)
 {
-	return UnmapViewOfFile(start) - 1;	/* 0: success; -1: fail */
+	BOOL ok = UnmapViewOfFile(start);
+	return ok? 0 : -1;
 }
 
 
@@ -494,7 +513,13 @@ int uname(struct utsname* un)
 
 	// node name
 	DWORD buf_size = sizeof(un->nodename);
-	GetComputerName(un->nodename, &buf_size);
+	DWORD last_err = GetLastError();
+	BOOL ok = GetComputerName(un->nodename, &buf_size);
+	// GetComputerName sets last error even on success - suppress.
+	if(ok)
+		SetLastError(last_err);
+	else
+		debug_warn("GetComputerName failed");
 
 	// hardware type
 	static SYSTEM_INFO si;
diff --git a/source/lib/sysdep/win/wsdl.cpp b/source/lib/sysdep/win/wsdl.cpp
index 23bfef14c7..d0a957e264 100755
--- a/source/lib/sysdep/win/wsdl.cpp
+++ b/source/lib/sysdep/win/wsdl.cpp
@@ -542,6 +542,9 @@ void SDL_Quit()
  */
 int SDL_SetVideoMode(int w, int h, int bpp, unsigned long flags)
 {
+	int ret = 0;	// assume failure
+	WIN_SAVE_LAST_ERROR;	// OpenGL and GDI
+
 	fullscreen = (flags & SDL_FULLSCREEN) != 0;
 
 	/* get current mode settings */
@@ -610,16 +613,20 @@ int SDL_SetVideoMode(int w, int h, int bpp, unsigned long flags)
 
 	int pf = ChoosePixelFormat(hDC, &pfd);
 	if(!SetPixelFormat(hDC, pf, &pfd))
-		return 0;
+		goto fail;
 
 	hGLRC = wglCreateContext(hDC);
 	if(!hGLRC)
-		return 0;
+		goto fail;
 
 	if(!wglMakeCurrent(hDC, hGLRC))
-		return 0;
+		goto fail;
 
-	return 1;
+	ret = 1;
+
+fail:
+	WIN_RESTORE_LAST_ERROR;
+	return ret;
 }
 
 
@@ -760,9 +767,9 @@ SDL_VideoInfo* SDL_GetVideoInfo()
 
 #ifdef DDRAW
 
-	static bool init;
-	if(!init)
-	{
+	WIN_SAVE_LAST_ERROR;	// DirectDraw
+
+	ONCE({
 		IDirectDraw* dd = 0;
 		HRESULT hr = DirectDrawCreate(0, &dd, 0);
 		if(SUCCEEDED(hr) && dd != 0)
@@ -774,9 +781,9 @@ SDL_VideoInfo* SDL_GetVideoInfo()
 				video_info.video_mem = caps.dwVidMemTotal;
 			dd->Release();
 		}
+	});
 
-		init = true;
-	}
+	WIN_RESTORE_LAST_ERROR;
 
 #endif