Once you become aware that a computer on your network has become a bot, take immediate steps to remove the malware from the computer.
The fastest and most effective way to remove malware is to install an anti-virus/anti-malware program and set it to automatically update. Popular vendors include Microsoft, Symantec, Norton, McAfee and AVG.
You can find a list of free botnet detection and remediation resources at www.stopthinkconnect.org/keepacleanmachine.