Spyware is any program that hides on your system with the intent of collecting marketing information about you and your surfing habits, and/or displaying pop-ads on your screen. Spyware is currently the most annoying problem on the Internet, and in my experience, the single most common cause of unreliability and slowdowns on Windows machines.

1. Start by killing any viruses and trojan horses. Boot to a CD with an up-to-date antivirus and disinfect the system, please see CreatingAnAntivirusBootCD for further details. You can also use online antivirus checkers like Housecall from Trend Micro, F-Secure’s online scanner, or Panda’s ActiveScan.
2. Then remove spyware using the free AdAware, Spybot S&D, and Microsoft’s anti-Spyware program. Update, scan and remove all the infections you can, then turn on the immunization feature of Spybot or the Real Time Protection and browser protection features of Microsoft’s program. This should take care of the spyware, but often is insufficient to remove browser hijackers.
3. Read up on browser hijackers at SpywareInfo.com. You may need to use a dedicated tool. Start with Hijack This to figure out the name of the infection. Common hijackers include Cool Web Search and 180 search Assistant. For a list of all the known Browser Hijacking programs and removal tools visit Sysinfo.org. Searching Google can also be useful in finding removal help.
4. I recommend using Mozilla or Firefox to avoid future hijacking. If you must use Internet Explorer install Spyware Blaster or Microsoft’s anti-Spyware program (recommended) to block future infections.

Spyware is not a problem on Macintosh computers. Yet.