If you have a Win98 Floppy disc and the XP OS is Fat32, You can start with the Windows floppy and Format with that. It should look like this after start with floppy
A:\>FORMAT C: hit enter and follow instructions on screen you do not need to put a label in.
If it's NTFS files System you will first have to run FDISK so that it would look like this
A:\>FDISK press enter