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.
Edited by Myke (01/03/07 08:20 PM)
_________________________
I swear, troubleshooting is a science: the best discoveries are always on accident...