Two Domains and One Laptop

I have a user that has a laptop (WinXP Pro) and usually resides in an area office. The laptop is joined to the Win 2000 domain in that office. Problem occurs when he comes to the main office. We have a Win Serv 2003 domain. In order for him to access the mapped folders on our network I need to join him to our domain. I would like him to be able to access our files without having to re-configure the domain. What is the best solution??