Search them out. Run a full security scan with your software and delete files that could pose a threat. Make sure your computer is set up to perform regularly scheduled scans and to automatically download security updates.
Reinstall. When all else fails, “reinstall your system from scratch.” If it comes down to this, be sure to back up your files to a USB hard disk and fully reinstall your operating system. Then copy the files back and reinstall applications.
Remain vigilant. Lock down your Wi-Fi using WPA security, and update your security software, firewall, and router. Continue to keep tabs on incoming and outgoing data via programs like Zone Alarm (PC) or Little Snitch (Mac). When a program displays unusual activity, check the Web to see if it’s known to be malware.
Source: Popular Mechanics