The computer seems to freeze a lot, sometimes it gets lines going through the screen, and recently the screen would turn black for 1 second, and then go back to the desktop for about 5 seconds, then turn black again, and it kept repeating this process until I reset. I previously had Windows 98 on it, and it did the same thing. I don't know what the problems is, and I don't know how to find out what the problem is. I just reformatted, I did the extended memory check, but thats about it. Does anyone have any clues on what the problem is... or any ideas of how to find the problem?
Since it happened before the XP upgrade we can rule that out.
You didn't mention noise... is there any associated sounds or noises such as fans winding up or down when this happens or at other times?
You didn't mention the power supply; what is yours rated at? (watts; ie. 250w, 300w, 350w...)
Since it seems to be centered around your display or video card performance, here are two more suggestions.
1) Try replacing the monitor with a different one. This is a bit of a longshot but relatively easy to experiment with. Additionally, take note of how the monitor is installed in system or display settings. Is it <default monitor> or something specific... is it a flat panel or CRT type.
2) I would definately re-seat the video card if I were you and inspect it for any obvious flaws or oxidations. give the card contacts a light cleaning while avoiding touching them with your fingers. And if you're going to open it up and go in you'd might as well give it a good dusting out and check and re-seat any other cards and the RAM as well.