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.

is it overheating, Are you a hot Climate!!
,shutdown,open the case ,check cpu fan for dust,the powersupply fan for dust ,leave the case cover off and if possiable place a table top fan so it blowes into the case .
If you know you way arounf the BIOS ,go in and check the section telling the Temp of the cpu and the case temp !

if the computer lights are still go then it sounds like your monitor may be taking the plunge... just a thought....... when the monitor goes black try things like Ctrl+Alt+Del+Del to see if it restarts the comptuter, if so then I would look into the monitor situation. Im an expert by no means so dont hold me to it ... hope it helps :mrgreen:

Have you tried turning the monitor off and back on? We've got a TV (same design idea) that loses all its color at random times, and I turn it off and back on and it fixes it.

I did a little test. I turned off my monitor and left the computer alone for a while. I came back, turned on the monitor and the screen was still black with the white lines. So the problem isn't the monitor. And to make a note... This problem happens after I don't move the mouse for a straight 15 or 20 minutes. I could be on the computer all day long and not have this happen as long as I move my mouse every 10 minutes.

Just incase it isn't clear... that little test I did... I was using the computer and it was acting fine- I was moving around the mouse and doing whatever. Then I turned off the monitor and left the room for about 30 minutes. Came back, turned the monitor on, and the screwy blank screen with lines came up... instead of the way the screen looked before I turned the monitor off.
So regardless of how the monitor is being used, my hard drive hates it when the mouse sits still.

As far as the fans and the temp. of the computer... the computer doesn't feel warm to the touch. If the fans weren't working properly would the computer feel warm?

Maybe... Maybe not. You're better off checking heat by putting your hand in the air current behind the power supply.

I'm not sure it's overheating, but if you only have a processor fan and power supply fan, it very well could be.

never hurts to see if there is an update for your OS.

Could there be a fault on the motherboard? - that thing with the mouse...

well I actually sat down and waited for my computer to act up. After 23 minutes a window came up and said that within 15 seconds your computer will go into Stand By (or some phrase similar to that). BUT... when I go to power options I have the monitor, hard disks, and hybernate all turned off. There isn't a spot for "stand by" in my power options window.
Am I missing something here??

And after the computer supposedly goes into stand by the funky lines apear and when I move the mouse nothing happens. When I hit keys on the keyboard nothing happens. And nothing happens when I hit Ctrl Alt Del. I have to actually hit the reset button in order to get the computer to turn off and come back on normal.