Also, try leaving it running for a day or two without shutting it off or putting it to sleep.

There are a few "behind the scenes" admin tasks that are run every so often, usually at off times, and if they haven't run in a while, they can fire up while you're using it, which will slow things down.

Also, how much disk space do you have free? Try to have at least 10GB free, and I'd recommend investing in a good disk defragmentor and run it every few months.

I've been using Yasu (Yet Another System Utility) to do maintenance & clear caches: http://jimmitchelldesign.com/projects/yasu/

Last two things I can think of - if you are using Time Machine or .Mac sync (MobileMe now), and have an older/slower machine, it can slow down when those tasks are running. I have a G5 iMac and the performance hit is apparent at those times. Though, it is a "slowing down", not an "acting weird" issue. Finally, if you have a family of dust bunnies in the computer, blow them out - don't want to interfere w/ cooling. These are probably not the cause but can't hurt to mention.

Isn't asking about virus protection for the Mac kind of like asking about a body guard for Jack Bauer?

Vacuum them out is a better option. Blowing into with compressed air can force particles into places you don't want them.

LOL.

There have been theoretical Quicktime, email and jpg exploits that could eventually be used to create zombie Macs, but Apple has done a good job of handling those potential threats in occasional OS updates.

Microsoft has made great progress with malware prevention since XP, as seen here:

http://tinyurl.com/cm3to9

Unfortunately, about 70% of businesses are still running XP. The net is that it appears there are about 100,000,000 Windows computers infected worldwide at this moment.