Cannot access internet

I have a 25 node LAN with all workstations running WinXP Pro SP2 and servers running Win2k3 R2 SP2. One of those workstations has been recently losing its ability to access the internet as well as the inability to ping the default gateway. Previous attempts to correct the problem were resolved by rebooting the router (SnapGear 300) and ISP-provided cable modem and/or rebooting one of the servers (which should make no difference as no outbound traffic is routed through any servers). All workstations have identical protection configurations: McAfee Security Suite with firewall activated; windows firewalls disabled via group policy; plus periodic scans with Spybot S&D.

Any ideas as to the cause and possible resolution? Thanks. Mdhubblecat.