The most thorough one I have used is
Super Antispyware. It found virus files that AVG, Ad-Aware, Avast, and Spybot could not.
Unlike most others it does not have to be installed into Windows to run. You still need Windows to boot and load but it does not require being tied to the Windows registry. That way you can install it onto a Flash drive and run updates. Then boot an infected PC into safe mode mount the flash drive and run the program.
This is really handy when viruses reroute proxies preventing other AV's from updating. If the program or installer will not run rename the .exe file as some viruses are clever enough to block some anti-virus programs from loading.
Don't spend too much time trying to remove the virus. Some of those buggers are very persistent. Occasionally making a data backup and Windows reinstall the quicker solution. When any of my customers have reputable and up to date Anti-Virus software and have been inundated with viruses I usually recommend this course to save them money.