Keen advice and free products to combat viruses and intrusions The definition among\u00a0security experts for a "safe" computer? One that's unplugged from a network, turned off, still in the box and buried in the back yard.While a touch pessimistic, the point is simple: Connected computers are not safe from intrusion or viruses. But don't despair, because there are ways to protect against the majority of virus problems. Let's talk about some ways to sidestep viruses through common-sense computing, and four free programs available for protection.First, a lecture on avoiding viruses. Don\u2019t ever open e-mail attachments from people you don't know or attachments you don\u2019t expect. Plenty of viruses spread by sneaking into e-mail address books (almost always Microsoft's Outlook or Outlook Express address books) and sending messages to everyone in the address book.Second, don't let people send you .Doc files via e-mail (viruses hide in Word macros). Microsoft Office (and Windows) tries to hide the extensions to files and forces you to save everything as a .Doc file, but you have a choice. Under the File menu in Microsoft Word (and most other word processors), choose Save As (or Save as Type) then choose Rich Text Format (rtf) or Text Only (txt). If you see an extension of rtf or txt on a file, the virus-free odds are in your favor.Make sure you have configured Windows to show file extensions, because a few virus criminals have added .txt to the file name to make the attachment look like an extension when it really isn't. To do this, go to Windows Explorer > Tools > Folder Options and clear the box saying "Hide file extensions for known file types."Another file extension to avoid is Exe. If you try to download something from a Web site and the system asks if it should open the file or save it to disk, say no to everything and hit cancel immediately. Run your cursor over the download link and see if the URL, including file name, appears in the status line across the bottom of your browser window. If it says exe, say no unless you know what it is.Finally, if you must accept .doc or .exe files from someone you don't completely trust, open them on an isolated computer. Download the file or e-mail, then unplug the network card from your computer before opening the file. If there is a virus, it may mangle your computer, perhaps forcing you to wipe everything clean and reinstall. That's a major pain, but far less a pain than reinstalling every computer on your office network.I can hear you saying that Christmas bills stare you in the face and buying quality virus protection for every computer in the office will cost too much. Au contraire. As a belated holiday bonus, I gathered four free virus protection options.The one I've been using is\u00a0AVG AntiVirus from Grisoft. It offers virus signature updates, scheduled and real-time scanning and isn't technically threatening. There are many other features to check out, too.Alwil Software offers Professional and Home virus products under its Avast!4 label. I've heard good things about these folks and their products, but haven't tried them personally. If you try them, let me know what you think.CommonSearch offers VirusCatcher or\u00a0Vcatch. This program focuses on files being downloaded by e-mail or Web pages.Last, McAfee Security offers a service called\u00a0FreeScan. You can run the scan program online directly from the Web site or download it and run in on a regular basis.