Eek, well. WSS lives mostly in IIS, so if it really wont uninstall nicely go ahead and blow away all WSS related IIS websites and app-pools. Then remove the directories associated with them. for me, these directories live in
C:\Inetpub\wwwroot\wss\
Please ask this question (and similar ones) in the administration forum and not here in the development forum.
That said, uninstall from the add-remove programs should do it, but if you have a problem uninstalling from there you can manually delete the databases from SQL, and the IIS web sites and application pools from the IIS console.