An Error has occured while connecting to w:

I am running a windows XP with SP1 workstation which is
connecting and logging into a Windows 2000 Domain
controller with no problems at all. If I leave the PC
logged in for an hour or so, and come back to use a
networked drive, I find that I cant get access unless the
workstation is restarted again.
However, I can still run the internet, which goes out via
a router - not server dependant, and also I can still
access the server remotely by Microsoft Terminal Services
CLient from this workstation, yet cant use any network
resources.
When I click on any mapped drive letter, the error that
comes up is:

An Error has occured while connecting to w:
\\Server\Shared Name
Microsoft Windows Network: The device name is already in
use.
The connection has not been restored.
thanks