Everyone will have one.. I can only give you mine with a little experienced tossed in..

1. Only have one antivirus program installed on the computer.. Some antivirus programs won't LET you install a second one and even in situations where you get a second one installed, because they tend to lock into deep system processes, whether running realtime or not, having more than one on the machine can cause conflicts and failures to correctly handle detections. I've seen it happen and cleaned up infections on a few machines where both McAfee and other antivirus programs were forced onto the same computer.

That said, if you ever need a "second opinion" regarding a potential infection on your computer, an online scan is a good effective way to get such..

2. If you're going to run a realtime antispyware program, much like the advice above, use only one running realtime.. As to having others installed on the computer, that depends on the program. Some just don't handle other antispyware programs well, but not most.. For many reasons, many antispyware programs have options to be used without running realtime. As such, they have the option to NOT hook into those deep system processes and sit idle unless you run a scan with them.. In addition, because I clean out badly infested computers as a side business, I've found that different programs find different things and it can change from month to month as to which one is best.. So, we keep a couple of different standalone antispyware scanners on all the computers we manage, and we run scans with them frequently.

Hope this helps.

Grif