Here's a list of free anti-virus programs for Windows, Mac and Linux. Most freeware is for home use only, with the exception of Microsoft Security Essentials. Please read the license agreements when installing freeware programs.

Windows

Ad-Ware Free Antivirus+

- http://www.lavasoft.com/products/ad_aware_free.php
Leaner, meaner, faster. Ad-Aware Free Antivirus+ combines our legendary Anti-spyware with a super fast, free Antivirus.

Avast

- http://www.avast.com/index
Free antivirus with anti-spyware protection.

AVG

- http://free.avg.com/us-en/homepage
Free antivirus and anti-spyware security software for Windows.

Avira

- http://www.avira.com/en/avira-free-antivirus
Keep your PC clean with Avira’s free antivirus, an award-winning virus removal and prevention software.

BitDefender

- http://www.bitdefender.com/solutions/free.html
A free antivirus that uses #1 ranked technology and is feather-light.

Comodo Antivirus for Windows

- http://www.comodo.com/home/internet-security/antivirus.php
Free Internet Security. Complete protection against viruses and attacks for your computers.

Microsoft Security Essentials

- http://windows.microsoft.com/en-us/windows/security-essentials-download
Use Microsoft Security Essentials to help guard against viruses, spyware, and other malicious software.
It provides real-time protection for your home or small business PCs.

Mac

Avast! Free Antivirus for Mac

- http://www.avast.com/free-antivirus-mac

Comodo Antivirus for Mac

- http://www.comodo.com/home/internet-security/antivirus-for-mac.php

Sophos Anti-Virus for Mac Home Edition

- http://www.sophos.com/en-us/products/free-tools/sophos-antivirus-for-mac-home-edition.aspx
Free antivirus for Mac.

Linux

Comodo Antivirus for Linux

- http://www.comodo.com/home/internet-security/antivirus-for-linux.php

Sophos Anti-Virus for Mac Home Edition

- http://www.sophos.com/en-us/products/free-tools/sophos-antivirus-for-mac-home-edition.aspx
Free antivirus for Mac.

In some cases your virus program might not be able to remove a virus because it's taken over control of your windows environment or you can't boot in to windows at all. Bootable virus scanners will take care of this problem. Most bootable scanners are linux based operating systems. Be sure to read the documentations before using any of these programs.

Generally how they work is, you download the .iso file and burn it to a CD or USB. Boot the computer from the CD or USB. Once your in the scanning environment, make sure to update the latest virus definitions. From there, just run a scan and wait.

AVG Rescue CD

- http://www.avg.com/us-en/avg-rescue-cd

Avira AntiVir Rescue System

- http://www.avira.com/en/download/product/avira-antivir-rescue-system

BitDefender

- http://www.bitdefender.com/support/How-to-create-a-Bitdefender-Rescue-CD-627.html

F-Secure

- http://www.f-secure.com/en/web/labs_global/removal-tools/-/carousel/view/142

Kaspersky Rescue Disk

- https://support.kaspersky.com/viruses/rescuedisk

Windows Defender Offline

- http://windows.microsoft.com/is-is/windows/what-is-windows-defender-offline
Here's a more in-depth article on how to load and run Windows Defender Offline.
Microsoft Security Blog - Microsoft’s Free Security Tools – Windows Defender Offline

Virus removal tools are small programs designed to remove specific viruses.

AVG

- http://free.avg.com/us-en/virus-removal

BitDefender

- http://www.bitdefender.com/free-virus-removal/

F-Secure

- http://www.f-secure.com/en/web/labs_global/removal-tools

Kaspersky

- http://www.kaspersky.com/virus-removal-tools

McAfee

- http://home.mcafee.com/virusinfo/virus-removal-tools

Microsoft

- http://www.microsoft.com/security/pc-security/malware-removal.aspx

Sophos

- http://www.sophos.com/en-us/products/free-tools/virus-removal-tool.aspx

Symantec

- http://www.symantec.com/security_response/removaltools.jsp

ESET

- http://www.eset.com/us/online-scanner/

Kaspersky Security Scan

- http://usa.kaspersky.com/downloads/free-anti-virus-scan

Microsoft Safety Scanner

- http://www.microsoft.com/security/scanner/en-us/default.aspx

VirScan.org

- http://virscan.org/
For uploading and scanning individual files.

VirusTotal

- https://www.virustotal.com/en/
For uploading and scanning individual files.

TrendMicro HouseCall

- http://housecall.trendmicro.com/
HouseCall can quickly identify and fix a wide range of threats including viruses, worms, Trojans, and spyware.

Eicar's virus test file.
At Eicar you can download a test file that consist of a harmless string of code that should trigger your anti-virus program.