Jeremy Sutherland asked if it's really a good idea to supplement your regular antivirus program with something like SuperAntiSpyware or Malwarebytes Anti-Malware. I've recommended doing just that in the past.
Good question. A lot of experts will warn you that running two such antivirus programs could cause problems. And they're right, provided the two programs are both resident. Resident programs remain running in the background, and resident antivirus programs check every file that comes into your PC or that you open. Having two programs constantly doing this is asking for trouble, or at least for a very slow PC.
(Some quick terminology: Modern antivirus programs don't just look for viruses, which are technically obsolete, anyway. They look for all kinds of malware. For historical reasons, always-on, resident malware protection programs are often called antivirus programs. For more on the confusing use of that term, see Is It a Virus?)
Every Windows PC should have one, and only one, antivirus program running constantly in the background checking for malware. I never recommended otherwise.
What I have recommended several times was that people scan their hard drive with another, non-resident anti-malware program once a week, and also if they suspect foul play. When the scan is done, you close the program and it goes away, without staying in memory. SUPERAntiSpyware and Malwarebytes' Anti-Malware are still my favourite choices.
Why do I recommend this procedure? Because no program is perfect, and it's good to get a second opinion.