If you already have Windows XP and it's taking all the drive space, then prepare for re-installing. Resizing existing partitions is possible, but can be tricky, and I would rather suggest to avoid it.
Do some extensive check on Google for related tutorials. What has worked for some on certain hardware can fail for others on different hardware.
If you just want to test Linux you might just as well burn yourself some Linux Live CD and boot from that. That should be enough for some hands-on-training with Linux and UNIX in general, without the need to change your existing PC setup.
The Knoppix Live CD has an option to save your custom data and changes onto the Windows XP drive as a separate and non-destructive bunch of files, so you don't have to start from scratch each time you boot into Linux. I suppose other Live CD distributions provide something like that as well.
This way you could also check out different Linux distributions, before making the final decision.
Hope this helps,