Quick one this, Microsoft have finally entered the security software arena with a completely free, lightweight, quiet, tidy anti-virus and malware package called, catch-ily, "Security Essentials".

It has received generally excellent reviews (http://www.winsupersite.com/win7/mse.asp, http://www.sitepoint.com/blogs/2009/10/04/microsoft-security-essentials-review/, http://www.thetechherald.com/article.php/200926/3926/Review-Microsoft-Security-Essentials etc.) and is the first Security product I have ever run.  I urge everyone to download and install it, unless you are a die-hard user of one of the paid-for packages, and arguably even then?

This won't be part of automatic Windows Update for a long while as the big anti-virus companies still carry enough clout to make rumbles about anti-competitive behaviour.

So download it from here: http://www.microsoft.com/Security_Essentials/

I recommend removing any other anti-virus package from your PC first, especially if it's a free one (AVG etc).