Avira offer a free antivirus that works really well so do Avast,
if it is free software you want I have a thread on the darksiderg that has links to hundreds of free (as in free beer) applications for you to download.
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
Free software from microsoft called Microsoft Security Essentials (MSE ) is safe and easy to download . AVG is another free download opting for the basic version.
The boy from Ballbrigan
April 2011
Microsoft Security Essentials from Microsoft website. It's a lot better than you think, and free. Un-install currently installed anti-virus program(s). The "(s)" may also indicate conflicts that are causing your problems in the first place. Also add Malwarebytes (free too) for removing the other stuff you tend to get yourself into, and set both up on update /scan schedules...good luck!