Vista X64 Steel Focus From Full Screen

I've been using Vista x64 for about 4 months now and in that time I have been having this problem where Vista sometimes steels the focus from fullscreen apps such as games. I run a game, the screen goes black for a second (which it does in most games before the intro), then the game minimizes to the system tray and I'm back at the desktop. If i then try to bring the game back to fullscreen (using the mouse or ALT + TAB) it immedially minimizes to the system tray again, some games even crash. In the "micro second" that the game stays maximized I can see that the game has indeed loaded correctly, cos if I maximize the game a couple of times I see that the game is proceeding with the load. Sometimes a restart helps, at other times a reinstall is needed. Some games I just can't run at all, and some runs fine all the time.

All drivers, firmware and the BIOS are of the latest versions. I completely removed my Nvidia display driver using "CCleaner" restarted and installed the latest one. I tryed removing the "focus steel" ability from Vista using a reg file I found some where (I forget where).