This sounds as if it could be a hardware issue.
So, with that out of the way, where are the hardware specifics such as make model and type of computer? Running XP Home/Pro and what service pack for software?
Help! This morning I started up my old Dell desktop that I use for school, which is about eight years old, with a 933 Mhz Pentium 3 processor, and 256mb of ram. So It's really old. Well, when I started it up, I didn't notice anything different, but when it got to the log on screen, everything was larger than normal. I went to the control panel, and it turns out the resolution was automatically set to 800 by 600, almost like it was in safe mode. The issue is, I know it didn't start in safe mode, but it won't let me change the resolution. I've restarted it several times, but it still does the same thing.