One of the secret pleasures of using a Macintosh is a by-product of the law of unintended consequences: with only 3% market share, we’re generally free of computer viruses. Apparently, most of the people writing these nasty little programs see the Mac community as being too small to generate enough attention to satisfy their egos. And that’s fine with me.

For the past few years, the best advice I could offer to people using Windows was this: Don’t ever open an attachment that arrives via e-mail that you weren’t expecting, don’t know what it is, or don’t know the sender. And if there was ever even the slightest doubt, you were better off deleting it. (If it turned out to be important, the sender could always send it again.) But now security experts are issuing warnings about a new variant of a common virus called Bagle that makes this advice obsolete. These new viruses – there have already been several reported – can infect your system without the use of an attachment — all you have to do is open the message to be infected.

These viruses exploit a security hole in Microsoft Outlook first discovered in October, 2003. This hole allows an HTML e-mail message open on your computer access a visual basic script online and run itself on your computer, installing the virus and infecting your system. The virus infects every .exe file on your computer, making it virtually impossible to completely remove it. It also works to disable many firewall and antivirus applications, a technique that is becoming more common.

The best thing you can do to avoid contamination is to be sure that you are running Windows Update regularly and visit the Microsoft site to download the latest security patches for Outlook.

Start here to look for the correct version for you:

If you have an older version of Outlook, then you might want tostart here to find the correct patch for your system:

Be sure to read the instructions before proceeding.

If you’re interested, you can read a detailed article about this problem at The Motley Fool.