Are you using any particular software when the freezes occur, or is it just random? Are you accessing files (i.e. in My Documents), when it happens? Also has your PC taken a bump or a knock recently, or have you had the case open for anything? When I built the PC recently for my mate, we had a freezing problem following the PC getting bumped in a car journey. I opened the case, and gently pushed all the cards to make sure they were secure, and the problem stopped, but I can't actually pin it down to a specific answer.