Continuous restarts without fully loading windows vista.

My nephew was using 2 of my machines and looking at his myspace page and you tube stuff. Now I have 1 win xp machine that stays on the windows loading screen and my vista ultimate machine constantly reboots without completely loading. I am suspect a virus because only the 2 machines that he has used out of the 7 I have here are having problems. I cannot even boot to safe mode on the vista machine. The xp machine has multiple accounts and I can boot to safe mode but cannot scan. Does anyone know of a virus that could be doing this is is this all just a coincidence? Thanks for your help.