if the mbr is knackered, the windows install cd will allow you to restore a default windows booting mbr
once you're there, you can then boot into linux with a live cd and attempt to repair grub manually (i can't advise on how to do that, as it's something I've never had to do)
if you boot the windows cd, then jump to the recovery console, log into your windows install and type:
that should restore the boot record to as it was when you first installed windows.
Then you can attempt to get your system dual booting again in however way suits you best.
I make no guarantee that this'll work for you, but you can be reasonably assured that it will prevent grub from loading.