Here are some great free anti-virus programs for windows users, beware it seems best you don't run two av programs at the same time, in fact it's not advisable to even have two installed at the same time.

AVG

AVG Free provides you with basic antivirus and antispyware protection for Windows and it is completely free.

http://free.avg.com/au-en/download-free-antivirus

The most downloaded software on CNET's Download.com
Free protection against viruses and spyware (antivirus and antispyware)
Fast, effective security that is kind on resources
Compatible with Windows 7, Windows Vista and Windows XP

Avast

Our avast! Free Antivirus often outperforms our competitors´ paid-for products and is the minimum protection anyone should have against viruses and spyware.

To run avast! Free Antivirus your PC must meet the following criteria.
Operating Systems Supported

Windows 7 (any Edition, 32-bit or 64-bit)
Windows Vista (any Edition excl. Starter Edition, 32-bit or 64-bit)
Windows XP Service Pack 2 or higher (any Edition, 32-bit or 64-bit)
Microsoft Windows 2000 Professional Service Pack 4

http://www.avast.com/en-au/free-antivirus-download

Minimum Hardware Requirements

Pentium 3 Processor
128 MB RAM
200 MB of free hard disk space

Please note that avast! Free Antivirus runs only on PCs with Windows XP and newer. Older Windows operating systems (Windows 95/98/ME/NT/2000) are not supported

Please also note that avast! will not run properly if you have other antivirus software on your PC.

Avira AntiVir Personal - Free Antivirus

Protects your computer against dangerous viruses, worms, Trojans and costly dialers.

The link below is a direct link to the download page and it will prompt you to download the file.

http://www.avira.com/en/free-download-avira-antivir-personal