Well, Try remembering when the reboots started happening and after you remember start to think of all the programs you installed into your system after that. (Tips Under this)
Tip 1: If your an XP User an easy way to figure out which software you have installed is by the Automatic Restore point creator. I always have this turned on. It creates a new restore point "Every" Time you install something.
Tip 2: This could be a virus or spyware that your Anti-Virus/Anti-Spyware is not picking up so try running an online scanner like
http://housecall.trendmicro.com/For Virus's And keyloggers/Other security risks, And if you think its spyware or certain types of adware try AVG AntiSpyware
http://free.grisoft.com/doc/20/lng/us/tpl/v5
If none of this helps it could also be a hardware problem :P,
Hope this helps
IHateTrojans