you can install up to 4 operating systems on a single computer due to the maximum number of primary partitions that the computer allows you to make. (also can make infinite extended partitions, but any OS is to go onto a primary for stability reasons.)
if all your hard drive is consumed with the C: partition in windows, then you either need to reformat your hard Disc (erase windows, re size partition [and add more partitions in the process] then install a fresh clean copy, all while using the win xp setup disc.
you could also try to have a Linux based re-partitioner program try to re size your c: but this is not guaranteed it will work as it is highly unstable and can corrupt windows data easily
after you have a spare partition on your hard drive, go ahead and change your Boot sequence in your bios or CMOS (if not done already)
to have the CD-ROM boot first with a debian disc inside it.
note)There are 2 versions of debian, the stable and the unstable. neither are useful as a Linux OS for beginners. The debian boot loader is, by default installed over the Win xp boot up files and if not done correctly, you loose access to your Windows OS.
Might I suggest Mandriva Linux (allot better off if purchase the paid beginner's version due to more hardware compatibility and automatic setup as opposed to the community version)