You might consider a system restore to 2 or 3 days ago and see if that helps. Did you install anything new prior to the first time it happened?

If it continues, right click "My Computer"-->Advanced tab-->Startup and Recovery Settings-->untick "Automatically restart". Next time it happens, it should give you an error code (something like 0x0000...etc.). Post that number and any text with it and we can go from there.


Otherwise, a format and clean install may be in order.