SUPERAntiSpyware is very good antispyware that is free for personal use. While Antivirus software is very important, Antispyware can help too in making sure that you aren’t getting hijacked by Spyware, Adware, Malware, Trojans, Dialers, Worms, and Key Loggers.
You can run it whenever you want, not needing to take up resources when you aren’t using it.