McAfee Viruscan 7.0 (this is what is on your office PC):
If you own any McAfee or Norton product you can combine $20 upgrade rebate from McAfee and a $30 Amazon.com rebate to end up with the product being free after rebates. Amazon.com is offering free shipping, so your total cost is $0 as long as you follow through with the rebates. (US, Windows).
This is another good virus protection product. You can get more info: Symantec.com
AVG is offering the personal version of their anti-virus software for free including free updates. The professional version is also free for schools and non-profit organizations. (international - see site for restrictions, Windows)