I've read most of this thread and it looks like a firewall issue to me (on the client).
Does Safe Mode with Networking allow you to surf?
I've also seen weird quirks with the Windows Firewall (especially on XP), in that you need to restore its configuration to default, irrelevant if it's enabled or disabled.
When one of our machines dropped off DNS, we opened up CMD through "run as administrator" on the client. That worked for us so we didn't have to worry about going any further down that road.
Originally Posted by mac_shinobi