I just had the same thing happen to my on my VIAO a month after my warranty expired. It's most likely a HDD error. Have you tried reinstalling Windows from scratch? If you do this and still experience the rebooting, then it is probably due to a physical error on the HDD.
Another question: do you ever hear strange (not normal) clicking noises coming from within the laptop? If so, then it definitely is the HDD; mine was making very strange clicking noises, almost like the HDD was stuttering, and then it would get stuck in a looping reboot.
You might want to think about replacing the HDD. If it's still under warranty, don't try tampering with it and just get it replaced free of charge.