A single license of McAfee VirusScan Plus 2009 costs around ~$40 but you may have it for free via this link**. Enter coupon code *** and click Checkout.
The code is valid through December 31, 2008 (no longer valid). You will also receive free upgrades to McAfee software for the next one year.
This release of VirusScan Plus is available for download on Windows 2000, Windows XP and Windows Vista systems only.
Update: You can now download free anti-virus software from Microsoft. It's free for life.