Multiple Domains

I have just installed SBS2003 on an existing network. So I now have 2 domains. All users now log onto the sbs2003 domain but need access to win2k files till 1st Jan. The problem I have is that I keep getting ‘multiple connections 2 a server or shared resource by the same user using more than one user name are not allowed’ the users still use there original user name and password. Thanks