Hope you figured it out...if not, maybe I can help as I ran into this same problem.
A little bit of info about my problem is that I have 2 hard drives (an SATA and an IDE...XP was installed on the SATA...IDE was set to MASTER...SATA you can't choose whether to make it MASTER or SLAVE). So, I put the XP disk in the CD drive...rebooted...chose REPAIR...fixed boot and mbr....nothing...still gave errors. I then opened the case and unhooked the IDE hard drive...rebooted...fixed boot and then fixed mbr...restarted. Worked! Then I turned off the computer, hooked the IDE back up, and rebooted...Worked! Now, a little more info that may be of use. I fixed boot first then fixed mbr second...It didn't seem to work right if I tried "FIXMBR" first and then did "FIXBOOT"....not sure exactly why, but, what the heck do I know...haha...It sounds like you are trying to repair the mbr first...I would suggest you go the other route...fix boot first then mbr.
Hope this helps...