Note: These “free” products normally are licensed only for personal home use.
Windows-based
free.avg.com/us-en/download
AVG Free Antivirus. A very good free antivirus. They have gone to a marketing model where at the end of the version, you must uninstall the old and reinstall the new. A small inconvenience for nominal free protection.
www.avast.com/free-antivirus-download
Avast! Antivirus is another contender in the “free for home use” antivirus market.
www.microsoft.com/security_essentials/
Microsoft Security Essentials. This product is not my favorite by far, but it is basic protection for your computer. It is of course free. Another bonus is that it is licensed for use on up to 10 computers, including small business use.
www.malwarebytes.org
Malwarebytes Antimalware. This is a top contender in the Spyware removal game. If you suspect you have a problem, download this one (the free version) and the following two after it in this list. A cocktail of several different cleaning utilities is the best approach. No one product is perfect!
www.superantispyware.com
SuperAntiSpyware Antimalware. The second part of your computer malware cleanup toolkit. Again, download the free version.
live.sunbeltsoftware.com
VipreRescue Antimalware. Very useful tool and the third part of your “malware cleanup cocktail”. Sunbelt Software is maker of Vipre Antivirus, among the best Antivirus programs I have ever used. If your computer is too infected to download and install Malwarebytes or Superantispyware, download this onto a working computer. Transfer it to the infected computer via CD-R or USB thumb drive. Run the .EXE file and it will extract and attempt to run. If it won’t run from within Windows, you may have to boot into Safe Mode (Command Prompt) and then run it.
Mac-based
www.sophos.com/products/free-tools/free-mac-anti-virus/
Sophos Antivirus for Mac. Sophos has been in the antivirus game for a long time. This is a good product and it’s hard to beat the price!
www.iantivirus.com/
PC Tools iAntivirus. Another Mac free antivirus contender.