HouseCall is a free antivirus program that does an online scan for viruses. Note: HouseCall is excellent for diagnosing if you already have a virus, but it will not protect you from getting a virus. HouseCall 7.1 recognizes malware by consulting their "Smart Protection Network" in the cloud.
To start, you download a small launcher utility that will download the latest components. There's no installation as such, but it does is store components and scan results in your TEMP folder. You can save the launcher and use it over and over and it will download newer codes if available. A full scan will take about 40 minutes.
One tester did observe that the malware on his test actually prevented HouseCall from downloading all its components so it couldn't remove any of that system's malware. Normally though after the scan is finished HouseCall lists the threats it found along with their threat type and severity level and for each threat it recommends what action to take - Fix or Ignore.
HouseCall is not a perfect solution so why use it? It can be used as a back up if you suspect your anti-virus program may have let something slip through. Another reason: if you are one of those people who don't keep their anti-virus software up to date (NOT RECOMMENDED, but I am surprised at how many people will invest hundreds of dollars in a computer and then refuse to spend a few dollars to protect it) or who use free programs, this is a good back up just see if you do have an infection.