Windows Security Essentials is capable software providing all the needed features including continuous monitoring of all the files on your computer, automatic updates of virus signatures and periodic 'off-hours' full scans of all your files. If you have a PC with no virus protection there's NO EXCUSE, all you need is a legitimate copy of Windows and an Internet connection to download, install and begin to use this software. (To download Windows Security Essentials click here.)
P.S. For many folks with software installed and working it's still cheaper at $30 or so to update Norton AntiVirus than to remove the software and loose both the working installation and the other valuable components that come with it. So you won't see us changing fast, but case-by-case well be encouraging some folks take advantage of FREE!
For a friend with no AntiVirus software, or any suspected problem you might use one of the FREE online virus scanning tools. Useful online virus scanners are available from Symantec and McAfee. (ç To take a look or to actually start a scan just click, an 'Active X' control will be installed)