XP reboot problem.

ok, so I just thought I would drop a note, we've got 3 machines here that all do the exact same thing. Windows XP, they reboot when you atempt to start them up. Haven't found a solution, think it's probably a bit more than coincidence that all 3 worked last night and do the exact same thing today. Hit up Symantec's website, didn't see any virus advisory, scanned with the latest Stinger, didn't find anything. Any dieas?

 

safe mode?