How To Password Protect SharedDocs over LAN

I have a laptop running Vista Home Premium and a desktop running XP home. The issue is, how do I put a password on the SharedDocs on the XP machine, so that when i access it over the network from the Vista machine, it requires a username/password. If I try to acccess Vista's SharedDocs from the XP, it requires a password. But how do I do that the other way around? I can see the SharedDocs on the XP from the Vista, but I want them password protected. The folder I want to password protect is the default Shared Docs folder. I do have an account set up on both pcs. Any ideas? Thanks.