Why can't a computer have multiple antivirus software at a time?

For instance, why can't someone have Norton AntiVirus, McAfee, AVG, avast, and NOD32. This should be a relatively easy question.

You can have as many as you want. Just don't have more than one running at any time or they will interfere with the work the other is doing . It's a simple case of that old adage, "Too many cooks spoil the broth".

One good one is all you need. If you want, run an online scan.

Of course, you can have an anti-virus and an anti-spyware program, or several anti-spyware programs, providing only one of these is running in real-time mode.

Having more than one anti-virus program won't add to your security. There is other software you can use which will. In addition to an anti-spyware program, use a Hosts file. Use SiteAdvisor and SiteHound. Keep *all* your programs up-to-date with the latest security patches. Use Firefox with NoScript. Use a firewall that doesn't leak. All this will keep you far safer than multiple anti-virus programs, but it's your pc, it's your choice.

First, it would be overkill and secondly, they have a tendency to interfer with each other as they use different patterns to find virus's.

Each one thinks the other acts like a virus so they fight each other http://www.andcomputers.info