The MBR cannot be in the wrong sector, as it is in fact a sector (the first of the primary drive). I didn't quite understand the error message from your description. If you have a Windows/DOS boot floppy, you could:
and then try again with the Mandrake installation. Alternatively, if you had a Live Linux CD (like Knoppix, Gentoo, SuSE or something), you could try to boot it and look for the lilo configuration file, to see if the problem is there. Finally, you could try installing Mandrake again, overwriting the previous installation.
BTW, you should always make a back up of your files when you do something like this. I've never once had a problem like this with Linux installations, but it is apparently quite common.