Unable to access internet

I have a Dell Dimension which has suddenly started using a Automatic Private Address (169.254...) for its IP instead of DHCP. DHCP is enabled. I have reloaded the driver and I have tried a different network card in the machine - no change. When I plug the cable into a different computer it gets an IP from my provider so I know its not the cable, my router or my provider. The last virus update (before I lost connectivity) was Aug 1, and when I do a full scan, it doesn't find anything. When I do an ipconfig /release the address gets released and set to all 0's. When I do ipconfig/renew i get a message that an invalid parm was passed (of course it doesn't say WHAT parm - that would be too easy). I think I've either got a bad registry entry or an corrupted IP stack (which I have reset without any errors being flagged). I'm out of options and patience. Any suggestions?