Reader Walter Leonard Jr. writes: I have read in a past column that there is no one security suite that will or can detect all security threats. I use Norton. Is it possible to utilize another like McAfee also? Will they run together without conflicts?

In general this is a terrible idea. Running multiple security applications at the same time, particularly anything that runs in real time like antivirus or firewall programs, can wreak havoc on Windows, crash your computer, give you many headaches, and, in the very best case, merely slow your computer to a crawl.

The problem is worst with antivirus programs, which is designed to keep constant tabs on files being changed or created without your knowledge. But since antivirus programs are constantly doing this kind of thing behind the scenes, there's a good chance one program will try to block the behavior of the other program, and vice versa. The result could be a catastrophic failure of both applications (leaving you without any protection), or just general instability in Windows. This is so common that if you try to install one antivirus app on top of another, many installers will warn you not to do so.

Even if you did find a combination of two antivirus apps that work together, would you really be more secure? Probably not. All reputable apps and even most smaller ones do a great job at catching recently released viruses, which are by far the biggest threat to your PC. An old DOS virus that one app doesn't catch, released 15 years ago and nowhere to be found? Probably nothing to worry about any more. Install the one application you're comfortable with and, most importantly, keep it up to date.

Now the caveat: This advice does not apply to anti-spyware applications like Ad-Aware and Spybot. Providing you're using them both only in on-demand mode, scanning your system whenever you request it instead of doing real-time protection (Ad-Aware's paid version offers this; the free one does not), this is not only OK, it's recommended. It's like having two sets of eyes checking over your homework instead of one, and you're twice as likely to catch anything out of the ordinary, in this case spyware.

So, to sum up: Antivirus and firewall (or all-in-one security suites), only use one. Anti-spyware, install all you want, two or even three are OK.