You would google EVENT VIEWER to learn how to clear those logs then cause the reboot then back to the EVENT VIEWER to see what any of the 3 to 5 logs contain. Now back to google to research what you found.
I have had this problem for the past year and a half or so...
Windows XP Home, SP3
No adware/malware or viruses
When I try to log into any one of the Safe Modes, the computer restarts several seconds after the login screen comes up, regardless of whether I just wait or click on one of the login accounts.. If I do it from MSConfig, the computer enters into an infinite restart loop. Rarely, if I restart my computer enough times and select Safe Mode, I am able to log in without a restart after these many attempts (but recent attempts did not let me do this).
Does anyone have any clue as to why this may be happening? I also tried "sfc /scannow," but that did nothing (maybe because I am using an SP1 CD with a SP3 system?).