Setting up a dual boot is hazardous business. You should back up all the important data in your Windows partition before you attempt to resize it to make room for Linux. Also you should make a WinXP boot diskette in case the Linux boot loader messes up MBR.
Here's a page that explains how to make a boot diskette for WinXP:
http://www.xxcopy.com/xxcopy33.htm
You can use a program called "QTParted" that is in SystemRescueCD
http://www.sysresccd.org/ to shrink your WinXP partition so that there's space in your hard disk for Linux. Remember to run the WinXP defragmentation program before resizing the WinXP partition. Try to evaluate the available free disk space as accurately as possible so that you don't accidentally destroy data in your WinXP partition.
Notice that you can install a boot manager called "GAG" from SystemRescueCD. It can be safely installed to MBR -- you can configure GAG graphically (without editing any config file) and it can be easily removed without any ill effects. This is a good thing if you decide that you don't like Linux and want to restore the Linux partition for WinXP. If you decide to use GAG, you need to install the Linux boot loader into the Linux root (/) partition (and NOT into MBR).
To try out Debian, I recommend that you use the Sarge beta installer:
http://www.debian.org/devel/debian-installer/
Read the documentation carefully, it will save you from many troubles later on.
