Ok...
Clean your whole computer if you havent done it in less than 3 months. Best way to do this is with something that blows air, so it wont destroy any part.
Why? Your computer might be dusty and it will make your computer slightly warmer, and if it reaches a temperature thats too high, there will be a restart.
Chance is high this solution will not help at all, but still try it!
2nd solution... Do you have enough hard drive space? 10gb is like the minimum you need for your computer to work normally. Anything lower than 10gb can cause freeze ups. So try to delete unneccesary things if you have them.
Chance is low that you have less than 10gb, but if you do, do what I just said!
Do you have any anti virus, anti spyware, firewall or register cleaner programs? If so, which?
Solution: If you think it might be a virus, just download another anti virus program thats pretty good. (Avast, NOD32(?), AVG, Mcafee, Norton, Windows live onecare etc.) If you have never run a register cleaner program, you might do so. A good one is registry mechanic, you can try it out for free, but you cannot fix any problems for free. But you can use it to see how much you have. Note that it does not support 64bit.
A pretty normal chance that the freeze ups are because of this.
Try to check the temperature of your CPU. above 65 centigrade degrees isnt good, unless you have A processor that can take up heat till 85 centigrade degrees or even 100.
Just try to feel at the vents of your computer to feel if theres coming out hot air. do not try it at the PSU fan, because most of the time it is hot, but try it at the fan under it (if you have one.) it should feel pretty cold, and not really warm when you are running something.
Well this is all you can try... I do not know the problem, but you can try these things to search the problem. Last time a reformat worked so I think it might be some kind of program you installed caused it.
TRY A SYSTEM RESTORE! (3 days ago?)