Investigate the "fixmbr" option online. There may be a few boot discs that can solve this. It didn't work when I tried the Windows Restore Disc but you should try that first (fix common boot problems).
I fixed it back on XP with a BartPE disc.
If you have a spare hard drive, there's a complicate option if you get stuck which involves creating an backing up the MBR file, then restoring it:
1) reinstall Windows (don't activate. No need to update.)
2) Install Acronis True Image (Seagate and WD have free versions. Must have one of their internal or USB drives attached).
3) Do a backup (can backup just the MBR)
4) Create a BOOT DISC for Acronis from within Windows.
5) reattach the original drive ONLY.
6) boot to the Acronis CD (BIOS boot order must have CD first before Hard Drive)
7) RESTORE the MBR only.