Something switching off the Ethernet port

My ethernet port has gone dead on me.

The event viewer shows a number of error messages at the time when ethernet
connection went down:

Warning 20/10/2007 9:51:52 AM E100B 4 None
Adapter Intel(R) PRO/100 VE Network Connection: Adapter Link Down

Warning 20/10/2007 9:56:38 AM Dhcp-Client 1003 None
Your computer was not able to renew its address from the network (from the
DHCP Server) for the Network Card with network address 001A925D41AC. The
following error occurred:
The semaphore timeout period has expired.. Your computer will continue to
try and obtain an address on its own from the network address (DHCP) server.

Error 20/10/2007 10:04:23 AM RasClient 20227 None
CoID={358EDDED-8270-4284-8E85-850A8B08F028}: The user HP_MULTIMEDIA\George &
Athelene dialed a connection named sympatico which has failed. The error code
returned on failure is 815.

This happened earlier in the year. To cut a long story short, I ended up
getting a new motherboard from HP and restoring the computer to factory
settings. This worked.. The ethernet connection worked for a week, then died
on me on 20/10/07.

I suspect the OS is somehow shutting it down.

I can still connect through the USB port, but I'd like to reactivate the
Ethernet port if possible and find out what is happening. With one proviso -
I'm not willing to restore to factory settings again. I've lost too much time
on the computer already.

Thanks

HP Pavilion Media Center m8020n TV PC
Bell Sympatico Speedstream 5200 modem
D-Link WBR-1310 router -> wireless to second (XP) computer
Windows Vista on primary computer (Media Center)