I was having a number of strange network issues on a laptop today. Here's the complete nuclear option for resetting your whole IP stack. This is for when "Diagnose and Repair" isn't cutting it. Thanks to JohnP for his help.
UPDATE: On Windows 7, you'll need to do this to reset your network adapters:
- ipconfig /flushdns
- nbtstat -R
- nbtstat -RR
- netsh int reset all
- netsh int ipv4 reset
- netsh int ipv6 reset
- netsh winsock reset
Now, reboot and pray. Possibly not in that order.