Fixmbr is not suppose to damage your MBR if you only have 1 OS install, it basically replaces the original file with a good one. It shouldn't that's what I'm trying to say BUT I AM NOT SURE!
Take the risk perhaps it may be the solution perhaps not but you'll never know until you try.
Your using XP Pro so your promt is C:\WINDOWS>
(of course C: being your harddrive letter which it would be if you only have 1 partition)