If your Windows 2000 or Windows NT 4.0 Server periodically loses connectivity with its' clients, and must be restarted, check the event log for:

Event ID: 2020
Source: Srv
Description: The server was unable to allocate from the system paged pool because the pool was empty.

-or-

Event ID: 2019
Source: Srv
Description: The server was unable to allocate from the system nonpaged pool because the pool was empty.

There is a good chance that this is caused by your anti-virus product.

Check with your vendor for the latest updates.