Does anybody know how to confirm positively that your PC (Windows or Mac) is not infected with any malware? It occurs to me that we don’t have a way of knowing. We can install anti-virus software which will look for signatures and perhaps some behaviors but generally doesn’t detect botnet software and other types of dynamic or zero-day threats. There are new services like FireEye that can detect botnets and traffic that is leaving your device towards known botnet sites, however these too are looking for known traffic to known sites and known behaviors.
I guess a clean install of the OS might lead you to believe your system is clean, but that assumes the install disk and image was not compromised which is generally true. However, an OS install is a bit extreme to validate a clean system.
So how do you know for sure that your system is not infected with something?