free.avg.com
(much beter than Norton that you need to pay for)
You also have avast.
Microsoft Security Essentials is also an option.
If you search a little, you can find several other that are also free.
Stay well away from anything that offers a free scan but ask you to pay to remove the detected treats. Those are false, or rogue, antivirus that are usualy trojans or scarewares.
Electro, April 2011