I'm actually betting drivers. It wouldn't be the first time I've seen an old driver not work after an SP3 update. Thankfully in this case, if I'm right, it'd only be the network driver. I'd roll back like Infection mentioned, update all drivers then try again. If you still have no luck, then I'd take a look at Tommy's post.
To save time though, I'd try Tommy's first as it's typically a five minute thing for the firewall, and a 1+hour fix to uninstall the service pack.