Random Client Disconnections

Can anyone help shed any light on a following issue I am having on a
Server that I manage.

It is an SBS2003 system will all of the latest updates installed. Set
up with 2 NIC.

There are 4 client PC all running XP SP3.

For reasons I cannot establish at the moment, a client will disconnect
from the server. Sometimes it automatically reconnect, other times
you need to double click the computer in the bottom right hand corner.

I have replaced all of the CAT5 cables and installed a new switch.
Even trying different wall sockets.

I have asked the users to note down times of disconnections, but have
not found anything in the Server Event logs.

What else can I try or where to look?

Regards

Trevor