Keeps crashing

I have the following setup:

Q6600 cpu
4gb corsair 1066 ram
4 x 500gb wd hdd's - all sata
2 x dvdrw
windows vista ultimate

I have been playing games - nfs uc and ut3 and every so often the game will crash. fine no problem - restart game.

but now the pc has started to crash for no reason.

one day it will run all day long no probs, other days it will boot and within 5 mins of booting will crash.

I have run several hdd diags, i run a vista diag, the ram diag, etc.

I thought it maybe a faulty hdd, so I have identified the c drive in order to swap this out for one of my spare 500gb's but before that I ran all the above tests and they have come back negative.

This install is only 3 weeks old after I got a c:\windows\system32\config\system error and had to reinstall the whole o/s again - after hours of fruitless searching on the internet for a solution that didn't involve a reinstall.

Any ideas people b4 I swap out the drive and do a fresh install???

Also, if you upgraded to IE8, this could be the problem. Remove it and revert to IE7