Number 7 on "My Top 10 Most Useful Free Programs" is AVG Anti Virus (Available at http://free.avg.com
AVG is a very useful and necessary program for anyone that is on the internet or checks email. There is a free version and several non-free versions available.
The non-free version usually come with some extra functionality or are part of a "Security Suite" but I usually do not recommend getting full suites of software as they are usually bloated with items you don't need.
AVG does run in the system tray and will let you know when updates are available to download and actually update itself automatically.
It also has the ability to run scans on demand.
Any client personal pc I come across that doesn't have any anti virus software usually gets this update. In fact I have even removed paid versions of anti virus software in favor of AVG.
I have seen many a client pc that is infected with several nasty virures. For the amount of effort that is needed to download and install this software I cant understand why anyone wouldn't be using it.