I have 15 clients running XP Pro on a single 2003 domain Server. Network
Drives have been mapped to each client.

If a client doesn't try to access the mapped drive when they first log into
the server the get disconnected after a certain period of time. The amount
of time it takes for them to be disconnected varies.

I have run the following:
Server side -
1) disabled the autodisconnect from command prompt
2) for the network card disabled the "Allow the computer to turn this device
off to save power" under the Power Management tab.

Client Side -
1) Regedit HKLM\System\CurrentControlset\Services\lanmanserver\parameters
and set the Autodisconnect to FFFFFF.
2) for the network card disabled the "Allow the computer to turn this device
off to save power" under the Power Management tab.

All machines, unfortunately have a public static IP address assigned to
them, corporate office requirement.

If the clients access the mapped network drives after the login the drives
will remain available to them for the entire day. But if they don't access
the drives they will be disconnected within four hours.

What is causing this problem and how can I get it to not disconnect the
clients?

Please, anyone out there with some advise would be greatly appreciated.

Thanks

I take it these are persistent mappings set in Explorer?

If so, I'd advocate the use of a logon script instead, with non-persistent
mappings. Generally much more reliable.

Other than that, you seem to have covered the usual causes.