I have no idea why, but after I leave my computer for about 15 or 20 minutes, the screen freezes. The screen goes black with 3 groups of skinny, close-together vertical white lines. The lights on my computer are still green and not orange, so it hasn't shut itself down. All I can do is turn my computer off and back on in order to get back to normal. I have the screen saver, sleep, and hybernate all turned off.
I can't think of anything I would have done to cause this. All I know is that after I came back from a 3 week vacation (probably the longest I've left my computer turned off) it started doing this. I can't recall any viruses that I've come across.
I have a Dell Dimension 4100, Pentium 3 processor at 800MHz, a 128MB 133MHz SDRAM, and I'm using Windows ME. And no, the hard drive isn't full.