Today, I just noticed that when I play videos in full screen, they are no longer full screen. They are now smaller and have black bars all around them. I do not have this problem with any other program (VLC, Quicktime, WMV, etc). Also, when I'm playing it without it being full screen, it gets bigger and looks normal. When I go back to full screen, it gets small again. I have two displays (primary: 1920x1200; secondary: 1280x1024). It almost seems like it is trying to use the secondary display's resolution for the video.
I tried reinstalling my gfx card drivers, as well as reinstalling iTunes and rebooting; the problem still occurs.
I ended up deleting all of the iTunes files in my profile (C:\Users\username\AppData\Local\Apple Computer\iTunes and C:\Users\username\AppData\Roaming\Apple Computer\iTunes). I then reopened iTunes and the problem was fixed.