Random Crashes In XP

I've had problems with my laptop recently and it has been back for warranty repair. To their credit it was only gone 8 days total but despite them running tests on it overnight they didn't experience the problem I'm having. Unfortunately for me when I got it back it is *still* happening. The laptop has 2x RAM slots and in each is a brand new stick of 512mb DDR400 since they suspected it might be a RAM issue. So far it happens at random times and is proving really difficult to pinpoint. Here's what I've tried so far and the troubleshooting I've done.

1. Reseat both RAM chips.

2. Try using both sticks of RAM, on their own, in each slot

3. Unplugged all external hardware (mouse, external HD, WLAN card)

For some reason if I only have 1x stick of RAM in 1 of the 2 available slots and nothing in the other I don't seem to get problems at least until earlier tonight. I've since switched the RAM stick out to see if it happens with the other but it hasn't yet. However, the crashes are so random I can have no problems for hours and then it suddenly crashes, other times it seemed to happen within a few minutes of getting into XP. It's a virtual clean install of XP with only drivers and updates installed.The only BSOD error message I've gotten so far is the following one, any others I see after I'll edit to the message. If you can help me at all I'll be VERY grateful, I don't particularly want to have to send the laptop off again if I can fix it myself. If it's a hardware issue though it'll go back.