Sounds to me like it wrote itself to the master boot record.
Go into safe mode command prompt and type FIXMBR, hit enter. Do not go into windows! Shut down the pc, pull the plug and then hit the start button, this is to make sure it isn't living in ram. Restart the pc, format the drive and reload windows.
Nasty is on the right path.... but fixmbr does not work in safemode... you have to do it from recovery environment... with windows xp its with a windows install disk(Just press R for recovery console) from their you can login and type fixmbr, After this you must type fixboot or you will not be able t "o boot up
In windows vista or 7, press f8 when booting... repair your computer(if you dont have that option then you have to use a full install disk and press repair your computer from the screen that has the install button on(down near the bottom) open the command prompt in the advanced options... make sure you in the correct drive(select C: or D: type in DIR to verify) then type bootrec.exe /fixmbr, then bootrec.exe /fixboot.