All you need a bootable OS disc. No serial number required. If you can find one, good, if not, you could torrent one. All you need to do is boot with the disc, goto the RECOVERY console, and i believe the command is fixboot, and/or fixmbr. Try the first command, if that doesn't work, FIXMBR, (master boot record). If neither work, the hardrive is probably HOSED
Thank you for your assistance with this problem I will try that.