We got blocked because we were listed in CBL for the following reason:
This IP is infected with, or is NATting for a machine infected with Torpig, also known by Symantec as Anserin.
This was detected by observing this IP attempting to make contact to a Torpig Command and Control server at 188.8.131.52, with contents unique to Torpig C&C command protocols.
The computer is in a NATted network connected to the internet with a dedicated IP. The firewall and DHCP logs pointed us to the right computer.
I scanned the computer using MBAM, Norman, TDDS Killer, Norton, Super Anti Spyware, Symantec Root Kit Killer and from Ubuntu with Clamav but none of them found a virus.
So, was my computer really infected? I won't take any risk and will reformat the computer, but this is really weird that a virus can exist on my computer and 7 AV products will not find it.