"So how can you tell if you’re machine is part of a botnet and what can you do about it? Statistically, Macs are safe from botnets, although not completely immune to all threats as we noted here. But if you have a Windows based machine, Prevx suggests you stay on the lookout for an Internet connection that seems inexplicably slow when you are online as it may be that a botnet infection is using your connection to send or receive data.
"If this happens, stop surfing, close your email software (e.g. Outlook) and try and open Task Manager by pressing the CTRL, ALT and Delete keys at the same time then selecting Task Manager," the company wrote on its blog recently. "When Task manager opens click on the Network tab and see if your PC is using the internet network connection, if it shows more than a few percent usage then this could be further evidence of something using your internet connection without your knowledge."" – Content courtesy of Is Your PC Part of a Botnet? – ReadWriteWeb