You could just manually go in and delete everything you don't need. With no XP disc in sight, you really are stuck with no other option than to buy a new disc if you want a fresh XP OS. So, if you want, manually go through:
Control Panel>Add/Remove Programs
- delete/uninstall any softwares you don't need or use
My Documents
- go through and delete all files/pics/etc. that you don't need
START>Programs>Accessories>System Tools>Disk Cleanup
- let it remove all junk and files that has accumulated if you never done this before.
Also, go into any other folder you have stuff you don't need and delete them. To go beyond all this, you should defrag your pc afterwards (same place as Disk Cleanup, but called Disk Defragmenter). A registry cleanup should help you out nice too since there will be many invalid paths after all those deleting and uninstalling.