No, it's not.
Thanks for your advice, I disabled fastwrites and I also ran a couple of hard disk surface scans but neither helped.
3DMark2001 ran for about 5 mins then it locked (screen went blank, high pitched noise from speakers, then rebooted).
I don't think it's a graphics card problem because only a minute ago I pressed the back button in my browser and I had a crash.
Strangely enough, I've been playing Doom 3 for hours with no problems at all.
Now that is one hell of a system test so I doubt if it's the graphics card or the power supply.
I've already proved this theory by using a GF4 and it still crashed.
Next thing to try is taking the HD from another PC and reinstalling Windows on that.
If it doesn't crash then it's either the Raptor HD itself or the SATA/RAID drivers.
If it does crash then it's almost certainly the motherboard.