I checked a computer at work that has XP + Office + a few other programs taking about 9GB - so that sounds way too high.
If you haven't already done so, back up all your documents to an external disc, download/backup drivers for your hardware and ensure you have discs for any software you want to reinstall as well as a valid XP key.
Then format your hard disc and reinstall: WARNING - YOU WILL LOSE EVERYTHING THAT YOU HAVEN'T BACKED UP TO EXTERNAL DISCS.
Depending on the XP disc you have it may be worth making sure you have an old Win 95 (or later) disc - "upgrade" discs like to see a copy of a Windows disc before proceeding.