Main Box Down Hard

My XP box is down hard. The message I read on the BSOD is unmountable_boot_volume. I need to fix my MBR, I think, but I cannot get there with the tools I have. My box doesn't have a recovery disk, and I don't want to format my hard drive to get it up. I do have a xp install CD from another box, but that CD only wants to format and install XP, alas, no recovery screen. What I think I need to look for is to make a bootable CD that has a MBR utility on it, but as I experiment more and more with this I am getting stymied. I'd appreciate anyone's help on this matter, as I have become this desperate.