XP is a bit on the greedy side.
There will be:
Restore points (there is a setting to restrict the number of these)
Temp files and folders for IE (or any other browser)
File swapping - how much is set aside for that? (if you leave that to XP, it gets outrageous) - again you can adjust that.
If you haven't told Outlook, or a few other applications (any MS office app defaults to C: ), to put their system files and user files on another drive, that will add to the daily clutter.
User settings get put there too, you can force those to another drive.
There are a load of other things that need to be managed, but I can't remember them off the top of my head. Make sure that when you install programs that you always select another drive for them because the C: drive is, again, default.
If you are really starting to hurt for disk space, you need to get a utility like Partition Magic that will aloow you to increase the size of the C: driev by stealing some unused space off another drive.
I had to do that about six months ago and took my C: drive from 10GB to 20GB. It has been quite steady at about 12GB since then, even with all the stuff above tweaked for minimum C: drive use.
A bit depressing, but I hope that helps.
ST