Have you tried checking all of the IP property settings to ensure they're set to automatically set to get them from the server/ISP? I know it's a stupid thing, but I've experienced on two separate occasions that workstations have "forgotten" that they were configured correctly and just needed me to run through the steps, not touch anything, and hit OK before they'd start working again.
Edit: Make sure you also open the advance settings and look at everything hitting OK without actually making any changes. If you do notice something that's off, it's a good time to correct it.
If you have access to a computer elsewhere, I'd also recommend downloading SP3 here
onto a flash drive or some other external device and just run the installer offline. It's possible that a simple update could correct the problem if it's a registry issue.
Edited by Ingle, 29 June 2009 - 04:24 PM.