Allow ONLY domain computers to connect

I thought one of the reasons for having computer accounts in the domain was that this was required to login to network resources.

So can anyone tell me how to prevent the following problem.

1) User brings personal laptop from home running XP
2) User already have a valid domain user account
3) User logs into personal laptop using local username and password
4) User types name of unc path.
5) Screen prompts for credentials
6) User types domainname\username and password

They can then access the F/P server from an unauthorised computer, bypassing domain login scripts and all security I have in place