I think you get a boot sector virus. Your MBR might be corrupted. You should:
1) Take out your hard drive and make it as a slave or as external hard drive (You have to buy the hard disk casing).
2) Backup some of your important files that you need such as from your 'My Document.
3) Reformat your hard drive and make a partition for C:\ max 20Gb preferably for windows.
4) Put back your hard drive to THE PC or Laptop.
5) Reinstall windows using the bootable Windows cd.
Good luck trying. I hope this will help you.