Windows 2003 Cannot Map to WinXP - No Logon Servers

I have a Windows 2003 Server as my domain controller, and a client running
Windows XP SP2. Both machines have shares set up.

My XP machine can map a drive on the server and read and write files. My
2003 server cannot map a drive to the XP machine because of the error

"There are no logon servers available to service the request"

The server can ping the client, and get internet access through the client
(it is identified as a gateway).

Can anyone suggest what might lead to this, and how I would correct it?

TIA

Charles