probably a corrupted MBR/boot.ini (cant remember which) or the entire disk could be bad. can you start in safe mode? does your bios provide SMART info on your drive?
if you have your windows install disk, boot from it and attempt to repair start up. if this doesnt work, might just have to reinstall and start over (not really bad anyway)
also: if your up to if, from inside the recovery window (when booting from the cd) you can go into a command promp mode and run a disk check. google it up.