Ad-Aware by LavaSoft has a Personal version that can be updated and run to purge a computer of most spyware. It is one of the better spyware cleaners, and updates easily. There are paid versions with more features, but the free version is a wonderful place to start, and more than enough for many people.

      If you use Internet Explorer then the Google Toolbar has one very important feature to help keep your computer clean, and that is a PopUp Blocker. Many PopUp Blockers tie up alot of computer resources, but Google's Toolbar is lean and efficient. However, it will not take care of pop ups that are caused by spyware or viruses residing on your computer. It will only take care of popups that are caused by websites that are visited. However this is not an issue on computers that are clean of spyware and virus. Best of all it is Free.

      As an additional level of protection against spyware ontop of LavaSoft Ad-Aware, on system that are running Windows XP and 2000, install Microsoft's Anti-Spyware and run they Real Time Protection. Another free product that will offer another layer of protection with very little performance loss.