Things to check for :
- Check if you have hamachi installed. ( Hamachi = Virtual LAN )
> Go to Start > Control Panel > Add/Remove Programs > Look for hamachi
* When you find it , uninstall it and then reboot your computer.
> When you try to log in again , it should work.
- Check if you are using another network adapter and not your main one. ( Eg: VM-Ware )
> Right click on My Computer(XP) or Computer(Vista/Win7) > Manage > on the left part of the list click on Device Manager.
* Look for the Network Adapter tree and click on the " + " to bring down the list.
* Look for other network adapter/s you don't use and disable them.
After disabling it , you have to reboot.
> Log back in again , it should work.