Mac OS X doesn't have a Product Activation scheme like Windows does, so no.
With that said, 10.0 is extremely old, and really not very good compared to later versions of Mac OS X. Why don't you get a copy of 10.3 (or wait a bit and get a copy of 10.4) instead? With the later versions there's an option in the installer called "Archive and Install" which will allow you to do a clean reinstall of the whole OS without formatting your drive, thus leaving all your files and data intact so you won't have to restore them from backup.