Just finished the install of service pack 2 and the PC won't connect to the net even with the modem

The light saying the router is connected to the PC is on but its constantly flickering on both the ethernet port and the router, it never did this before so my guess is the on-board ethernet port is fooked

I know the router is working absolutely perfectly as I just plugged it into this PC and connected to it straight away, same router port and cable so its defo something with the PC

I've run out of ideas now