Well I didn't think I'd be typing this any time soon, but I think it's fixed! My wife said it restarted last night when she installed her Photoshop and it asked her to restart the computer to complete the installation. Anyway, this morning I decided I was gonna open up the case to take out a TV card thats in there that we never use. It's the one thing that hasn't been tried, so I thought I may aswell give it a go. So I took it out, then powered up the PC, expecting it to make no difference. But there IS a difference - I have now restarted succesfully 5 times in a row! The most I have ever got to is 3, which was last week. I don't wanna tempt fate but I think all is now well.
The only downside(if you can call it that) is that I don't quite know what exactly has fixed it. I didn't check it this morning before taking out the TV card, so it could be somehow it fixed itself yesterday as my wife said it restarted for her. All I done the night before was install the latest drivers for my 6600GT, and I can't remember exactly but I donn't think it did restart after that. Maybe this TV card is the culprit, I may have to put it back in to check. Fingers crossed this is the end of this problem, I'll come back later to let you know if it rears it's ugly head again or not.