Games won't run in fullscreen mode, shrink to taskbar

I'm going mad here trying to solve this one. I'm new to Windows 7. I've just installed it 3 weeks ago, and I'm still in the process of installing all my software and small mini-games (shooters, arkanoid-type stuff, bejeweled, etc). I've had virtually no trouble getting my applications to run properly, but I'm having major difficulty getting my games to run.

While many do run fine, a major problem I'm having is that many of the small games I'm trying to run won't stay in fullscreen mode. If the particular game has an option to run in a window, it runs, but if I select fullscreen, the screen goes black for an instant while the game tries to switch to fullscreen, then it goes to my desktop, and the game ends up as an icon in the taskbar. If I click the icon, it comes up and tries again, and the same thing happens.

I've tried switching resolutions (my monitor is 1680x1050 22"), tried using my Nvidia control panel to use different scaling methods, and I'm getting nowhere. I'm running as an administrator with UAC disabled right now. My graphics card is the Geforce GTX275.

It's weird because it's only some games that won't run fullscreen, but it's quite a few, and some run in windowed mode, so I know Windows 7 can run the games. What could possibly be happening?