Mapped Drive Disconnects over VPN

We have XP remote clients connecting via VPN with a mapped drive to
the Windows 2003 Server at the main office. The remotes use the same
domain login credentials for the mapped drive and are not members (as
Users or Computers) of the domain. Periodically, at random, the mapped
drive disconnects and they have to login again. The VPN tunnel does
not disconnect. The remotes to not reconenct the mapped drive on
reboots either.

Should we need to add the remote users to the AD on the DC to avoid
this or is there something else going on?

Thanks