Sadly, there is alot of things that could cause this. Firstly, right click on my computer/properties/advanced/startup and recovery/ and un-check automatically restart.
That way, if it gives you a blue screen you will actually be able to see it makes things easier to track down. I would start with that, and then go from there.
How much memory do you have? And how many sticks? If you only have one stick swap it to another slot. If you have multiple sticks take one out and see if you get the crashing and error messages. If you still get the messages, rinse and repeat. Swap memory into different slots etc., That will rule out memory.
Also, if you have the ultimate boot cd there are several diagnostic programs on there that will help you out. You can get that cd here.
http://www.ultimatebootcd.com/If it still does it, start taking pci cards out, trying them in different slots as you did the memory.
Its a pain in the ass, but there is so many things that can cause the same symptoms.
It could possibly be a power supply issue as rtr said. What size power supply do you have? Theres a chance it may not be strong enough, but my knowledge of newer motherboards and power requirements are thin so hopefully somebody else can pitch in.
Oh, did you reformat your hard drive or just repair it? If its a software issue best thing to do is back up everything you don't want gone and reformat and install windows fresh.
