If there is no boot loader installed in the MBR simply boot off a windows xp cd and format and reinstall windows.
If there is an mbr all you need is to boot from a dos or freedos disk and use the fdisk /mbr command to wipe it out. After that the windows installer can do the rest
The Windows installer and Linux partitions don't mix. You first have to remove the Linux partition and then allow either the XP installer or another program to create the new primary for XP to install onto. What you missed there is that the XP installer can't see Linux or VFat12 partitions by itself.