This need to be confirmed by advanced user obviously, but there are probably two cases :
1. You are using only one hdd
Assuming u are using lilo, if you reinstall XP you may find that the LILO boot prompt no longer appears. XP is probably writing on your master boot record during installation.
Prepare a linux boot disc in order to reinstall lilo just in case. ( Keep an eye on loadlin as well )
2. You are using 2 hdd
In my case, i do have linux on hda and xp on hdb.
When i did my xp reinstallation, i simply disconnect my hda drive and reconnect it right after the xp installation.