Random Computer Restarts In Windows 7 64bit?

Over the past few days, my desktop computer has restarted twice randomly. Both times have been during times of low demand on the computer. The first time on 7/20/12 I was browsing the internet. The second was on 7/21/12 during a time when I left the computer idle on a browser for a phone call and came back to find my computer restarted with no information about windows updates or anything that I would expect to cause an automatic restart.I have not run into this problem before so I checked a few things which I thought may be a problem. First I did a virus scan because I have had a few problems with infections in the past. I ran several virus programs including malwarebytes, hitmanpro 3.6.0 (trial), microsoft security essentials, and spybot. After the first restart, I found a bootkit with hitman pro (Master Boot Record (Sector 0) in C:$MBR) which was deleted successfully according to the program.To be safe, I restarted and reran all the virus scans and found no further infection. I restarted another time in safe mode and ran them again and still found nothing else. However, the computer restarted today for a second time despite this infection being cleared. I reran the virus scans and found nothing again.Additionally, I loaded a hardware monitor program to check my computer temperatures and found them to be quite comfortable. My video card idle is running at 45C and my processor is running at 28C. I am running windows 7 64 bit which I installed from a disc on a desktop with an AMD Phenom II x6 1090T 3.20GHz processor, 8 GB RAM, and NVIDIA Geforce GTX 470 video card.