You're problem is more than likely caused by the virtual picture that XP takes of itself. This picture allows you to roll back drivers and "reboot to last know good configuration." This takes up valuable space on your c: drive.
Make sure you can display hidden files/folders.
Head to your desktop, then right click 'my computer', select "system restore", then turn off system restore. This will clean out your restore bin, then turn it back on and you will clean up a lot and reinstate your system restore for possible roll backs.
But just make sure your machine is running fine before you go deleting these files.