There are many different Anti Virus programs to choose from. Some of them are very expensive and some of them are free for home use. Some companies offer both, a free version and a paid version which will provide additional features.
Your computer may have come with a package from Norton or McAfee with a 90 day or a 1 year subscription to their updates. If you do not renew your subscription, you will stop receiving updates . If your anti virus is not up to date, it will be ineffective as it will not be looking for new viruses.
The Anti Virus software that I use and recommend is AVG Free. It does a good job of protecting your computer while using a lot less of your computers resources. AVG also offers a Pro version that you have to pay for that includes more bells and whistles than the free version, but it is still cheaper than Norton or McAfee. There is a comparison chart on the AVG download page. If you are interested in purchasing a Paid version of AVG for more protection or in a business setting (not eligible for free version), contact me for a discount.
Other free Anti Virus software that I have heard good things about include Avast Home Edition, ClamWin and Avira AntiVir.
It is extremely important that you have some type of up to date virus protection running at all times. While having more than one program to do manual or scheduled scans is a great idea (one may find what another misses), I do not recommend running more than one real-time virus protection program at a time.
Tools to remove viruses already on your computer!
These are downloadable or online scanners that you can use to check your computer for problems. Sometime one program will find threats that others have missed. Even if they cannot remove the virus, they may be able to indicate what virus it is so you can determine the steps needed to remove it. A search with your favorite search engine will usually point you toward tools or procedures for eliminating a stubborn virus.
ClamWin portable version - can be run without installing or from a thumb drive!
F-Secure Online scanning tool - you must use Internet Explorer 6 or later!