The master boot record sits in the some 100mb at the very beginning of the drive. The "fdisk /mbr" command repairs any changes to the boot record. You must have installed XP on a Fat32 partition for you to be able to even access anything on the drive there. You will need a 98 boot floppy to boot from and run the command at the dos prompt.
If you don't have an old startup floppy you make up a new one by downloading what you need from
http://www.bootdisk.com/bootdisk.htm