there are a few options, but the following are the one I would consider :

1. kerberos
2. ldap
3. samba

You can even use all three of them together to get what W2K(or Active Directory) provides.

I believe you would need (3) anyway for file server so if you don't have other security requirement, just use it as its authentication feature as well. Add LDAP on top of it if your requirement is more complex and then kerberos if you are paranoid.