The best way to combat this is to use tools, such as Sin Espias, that specialize in detecting Adware (invasive advertising), Spyware (data theft), and Trojans (external takeover and control of your computer), in combination with a traditional antivirus software. Most of these critters are harmless, but they definitely affect your computer's performance and internet connection.
Sin Espías detects Spyware, Malware, Adware, Dialers, Keyloggers and other potentially dangerous applications. It's very easy to use: you can scan all active processes on your system or perform a complete hard drive scan; plus the program updates automatically using the Internet.
Only checks for Spyware, Trojans, Backdoors, Dialers, and Worms.