Why you should use more than one anti-spyware program on your PC

PCWorld.com - Can You Trust Your Spyware Protection?

There are many anti-spyware programs out there like Ad-aware, Spybot S&D, Microsoft Anti-Spyware and Counterspy. Some of these well known and well regarded programs are being pressured by spyware companies to de-list their programs so that they won't be removed from computers. This means that if you use only one program to check for spyware you run the risk of missing something that is indeed spyware but has not been marked as such by your particular program.

I often find that Spyware S&D or Ad-Aware finds things that aren't picked up by other programs and vice-versa. Unfortunately, this means that you have to fill space on your computer to keep off things that you don't want - at least until Microsoft gets a better operating system.