Games won't play fullscreen. Stuck in center. won't expand. HELP!

Here's the gist of it. I've played two games within the last couple of days.
Normally, my games play fullscreen to the edges of the monitor with no
problems. I started a couple of games yesterday and now they will only play
in a small part of the screen - it's basically like the option to make your
wallpaper centered on Windows XP. In fact, it's exactly like it, but without
the option. How can I fix it to where all my games run full screen the way
they should. I run Windows Vista Ultimate with 2GB of RAM. HELP!