First post here - was trawling through forums in search of unrelated work computer probelm and v. impressed by quality and thoroughness of advice

Basically, due to the constant shutdown issues I'm having, my reliability index is at 2.83, on a brand new system purchased last november from Cyberpower in the UK. The main reason for this is that the system simply won't shut down properly - windows unloads and the screen goes blank (but not off - it is still getting an input from something), but the fans keep spinning, and if I leave the computer, it will eventually restart. Only way to turn it off is to manually hold the power button for 4 secs. I know it's not a mobo wiring issue, since if I turn the computer on and then push the 'off' switch before Vista loads, it turns off immeidately. It also turns off normally from within Vista booted in safe mode - so I figure the problem is one of the drivers being loaded - but I can't figure out which one it is. Problem started when I updated 650i mobo chipset drivers, so I rolled them back, but the problem stayed. I've gone into the startup options, and also tried disabling each of the items which starts one by one, then rebooted between, but none of these seems to be the problem. System is clean form spyware and virus apart from that, and regularly defragged / kept clutter free as well.
Anyone got any ideas which drivers might be causing this problem? I'm stumped....
Also, a quick question: when I close certain programs, the program closes OK, but then I get the Vista message that 'x stopped working. Would you like Vista to search for a solution?' (or something similar...!). This mainly happens when I'm using anything using the HL2.exe program (ie team fortress 2 and I think counter-strke source as well), and also whenever I close iTunes! Any ideas what this is, why it's happening, and how I can stop it?
Thanks in advance. If you want any more info, I have posted this issue with the PC builders in their forums (without getting it resolved from them) Here!
Cheers,
Wid.