Today I received a PC of a customer with a huge history of virus and spyware infections. It was impossible to connect the PC to a network.

I ran AVG Antivirus Free and AVG Antispyware Free to remove the remainders of the unwanted software, but still no network access. When I tried to ping my router I got some strange characters in the output window.

As usual, Google served the answer and pointed to Microsoft Knowledgebase Article 817571. Method 1 restored the network functionality.