If you're like me, you use e-mail or download files from the Internet pretty regularly. You're therefore at risk of letting a virus into your computer, as I recently did when I opened a message from a colleague and unbeknownst to me, infected everyone in my address book.

I was lucky; that virus, though annoying, was harmless. In fact the majority of viruses are not much more than harmless e-graffiti (commonly known as "hoax viruses"), ways for hackers to prove that they could have disabled your computer system had they so desired. The real dangers are the viruses with both bark and bite, capable of crippling your operating system or wiping crucial files from your hard disk.

Anti-virus software was developed to combat these viruses. Once installed, it runs constantly in the background of your computer, checking alien files and media like web pages, CD-ROMs, diskettes, and e-mails against the virus definitions incorporated into the software to detect any matches.

But the key to anti-virus software is in the updates. According to IBM's web site, at least six new viruses are created daily. Software will require regular updating-as often as once a week. The good news is that the cost of keeping up is paltry: a few dollars a year if anything.

You'll find anti-virus software where you find the other software your office uses: online software retailers, catalogs, or computer superstores. There are a few leading brands; you'll likely be choosing between software from Norton and McAfee. But what should your criteria be?

First off, low-maintenance updating is key. The best packages allow automatic, click-free updating; the software periodically retrieves the latest virus definitions from its web site and incorporates them into your software.

Also, you'll need to decide how you want your files to be protected. Can the software remove zipped viruses from e-mails without destroyed the entire document? Will it warn you of infected files before they're downloaded?

Once you make your decision, you may be tempted to download a free demo. We don't recommend it, for a few reasons: First, demo software does not contain the latest updates, and once you get to the point of requesting your first update, almost all companies will require payment for the full software anyhow. Second, most demos do not contain the automated updating features that ensure worry-free virus protection. And lastly, anti-virus software is very inexpensive - only about $40 per copy.

Since anti-virus software will be continuously running in the background, be aware that some programs can cut computer performance speed by up to 6%. If maximizing processing speed is critical to your business, carefully evaluate how much loss in speed different packages will cause, and consider adding more RAM to your computer if need be.

With multiple computers on a local area network, it's best to centralize anti-virus administration to a single location using network-server software that will configure, monitor, and maintain all anti-virus programs on the network -- even those from different anti-virus packages. Again, the top software packages are inexpensive, costing about $60 per computer.

With the right electronic vaccines, your computers can be virus-free.

Quick tips

Back up! No anti-virus plan is foolproof, so be sure back up regularly - and make backups of backups. Avoid pirated software. It's cheaper, but also more likely to be infected. For my eyes only. Network administrators should grant employees write privileges only to necessary areas. Since viruses can only do what the user who executes them can do, this practice will prevent the spread of infection. Turn it off for installation. When installing other software, make sure you disable the anti-viral software first, and then reactivate it when you're done.