Avast! is a free* for personal use real-time anti-virus package.

It scans various things for viruses, such as your hard drive and other forms of incoming data from the internet.

It does not have the best detection rate on the market, but it is the most complete and functional free anti-virus on the market, far better than the other free alternatives.

For your free download visit:
http://www.avast.com/eng/avast_4_home.html

*note: registration of the software may be required, but it is completely free