There is an app on my system called 'install-mbr', which may do what you want. The standard MSDOS MBR is apparently non-free. If you have your original MBR I believe you can whack it back on the disk with
dd if=my-old-mbr of=/dev/hda bs=512 count=1
Or if you can boot into DOS, then fdisk.exe /mbr.
As a rescue disk maybe you can use your original installation CD?
Please don't take this as any sort of expert advice! This is vague recollection + 2 mins on Google. Before you do anything drastic, consider how much your data is worth, and see if you can't beg, borrow or even buy a second HD to copy your files onto.
Edit: Uh-oh, upstaged while writing my post. Looks like better advice too