1. See if you can get a System Recovery disc from the maker.
2. Find an XP disk and see if it will do a repair install
3, Find an XP disk for a clean install: either a retail version with an unused license key, or one exactly the same as the disk you lost.
4. Back to your latest working backup image.
5. Buy another OS (Windows 7 if your machine is up to it).

Kees