Here are a few questions I’ve received recently from public like you, and my answers. I have edited and restated the questions a bit, for readability. that week my mailbox restricted questions about running antispyware software on a Mac, where to download free shield software for Windows, and viewing PowerPoint files with a free program from Microsoft.

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Do I need antivirus or antispyware software on a Macintosh running the Leopard operating system?

The Macintosh isn’t inherently invulnerable to malicious software. In fact, last week it was reported that there is a new scam on the Web that can plant a malicious “Trojan horse” program on the Mac. However, that is a rare event. There have been practically no viruses, spyware or other malicious programs written for the Macintosh that have actually spread outside the laboratory.

For that reason, most Mac users don’t run shield software, and defense software companies don’t form much of an effort to sell it for Macs. I don’t believe it is essential, so far, for all except the most paranoid (and those who run Windows on their Macs). In fact, freedom from the burdens of running and updating shield software has been one of the Mac’s big advantages.

Even the new Trojan Horse apparently relies on tricking the user, rather than on sneaking through holes in the Mac operating system. According to reports, to get infected you must go to a pornography site, and agree to download a program allegedly needed to view the porn. Next, the Mac will require that you type in your administrator ID and password to complete the installation, thus agreeing to install the program a second day. whether you do all that, you get a program that supposedly redirects your Web browser

to bogus Web sites. My advice: even whether you frequent porn sites, don’t agree to download any programs from them (that goes for Windows users as well).

Some interpret the appearance of that new Trojan Horse as a sign that the Mac’s increasing market share will start to attract a flood of viruses and spyware, and that Mac users will soon have to start running defense software. whether it happens, and the threats are more insidious than the latest one, I will be ready to change my recommendation. But not yet.

In reply to a question last week, you mentioned that there are free protection programs available for Windows. Could you please propose where to download such alternatives?

There are a number of them, but ones that I like are free, basic antivirus and antispyware programs called AVG from a company called Grisoft, which plus makes more elaborate shield software. You can download these programs at free.grisoft.com/doc/5390/us/frt/0.

I don’t have Microsoft PowerPoint but I occasionally get PowerPoint files as mail attachments. Somewhere I read that one can download a free PowerPoint program so you can read these files. Can you help me with that, please?

I don’t know of any free versions of PowerPoint, but Microsoft does offer a free program that will let you view, but not create or edit, PowerPoint files. You can download it here.

You can find Mossberg’s Mailbox, and my other columns, online for free at the new All Things Digital Web site, http://walt.allthingsd.com.

Write to Walter S. Mossberg at mossberg@wsj.com

Orginal post by Walter S. Mossberg