This page is dedicated to what to do if you need to reinstall either Linux or XP. Of the two, Linux is probably easier to install because of the way XP comes as an image. The most important thing that you need is a GRUB boot floppy. Make at least one of these. This is even a good idea if you don't use GRUB.
This is a simple matter of inserting the Debian DVD, and then following the whatever process you used to install Linux initially. Note that any of the partitions other than the first can be resized, so you have freedom to resize swap partitions. etc.
Provided that you do not wish to alter the first partition's size, then reinstalling XP is also relatively easy. However, note that you will need a GRUB floppy. If you do wish to resize the XP partition, then you will need to do everything again - all of the other partitions will get trashed.
If you do resize, and it fails to boot at any point (GRUB error 22), then insert the GRUB floppy, and follow the instructions for booting XP from floppy.
Once XP has been installed, you will need to put GRUB back on the MBR.