I constantly get this BSOD error on my Thinkpad X40. I have run hardware diagnostics on the RAM and it has come up fine. I have wiped the hard drive and reinstalled XP Pro aswell but this hasn't helped. I also have this machine as a dual boot with Ubuntu but do not get any problems at all with this which is where the problems lies.
If it was a hardware problem then I would have expected something to show up in the RAM diagnostics and for the problems to have been seen when running Ubuntu as well as XP.
It's worth adding that it originally just had the onboard non-removable RAM which still gave me BSOD errors but I have since upgraded the RAM with an extra 1GB a couple of weeks ago and still get the errors.