[niteshs]

When I boot up to Win XP the screen is OK, but after a couple of minutes, the screen turns into a mess (vertical bands of random colours ) Sometimes I can still make out what I behind it, but only enough to work out where the shutdown menu is.

My CPU (amd 2500+) and GFX (9700pro) are not overclocked. Is it a virus? If so it is not detected by McAfee, AVG or PrevX (all of which have been updated). My Radeon drivers are the most recent.

[brickout]

Since it doesn't happen right away, it sounds like a heat issue. Make sure the heatsink on your video card makes good contact with the die. Consider remounting it with AS5...

I had a card that did that, and removing the heatsink revealed a white substance that looked and felt like chalk...Applying good paste made the difference.

[Yuriman]

I have had about 3 fans fail, out of 5 or 6 Ati cards I have owned. Of course, these were all from the Radeon 8500 era, but make sure your fan is spinning.

[niteshs]

I just checked and it is spinning. (Incidentally this system has been working fine for the last year).

The CPU is 47 C and the system is 31 C.

Is it anything to do with AGP fastwrites? I've just disabled it on my card and board so I hope that will help.

[niteshs]

Turning off fastwrites has helped - computer has been stable for 40 minutes touchwood, 35 min longer than usual.

If it happens again I will switch down to 4x AGP mode.

[shellshock]

It has to be to do with your video cards memory. when you oc too much ( I know you said that you arent, just an example) you get lines of colors. you also get when theres a proplem with the memory chips. Put your finger on the chips and see if there hot. If so, you might have a heat issue, or your card could be failing

[IWasHungry]

I get similar issues when I OC my 9600XT's RAM too far, or set it for low latencies. Chances are it's a heat problem like the others mentioned.
RAM sinks are cheap. It couldn't hurt to put some on (And who knows it might even OC a bit).