First of all, Macs get viruses

too. In fact, my best friends MAC completely died, TWICE. My PC has not died once so there!
Second of all, my computer was doing the same thing. I knew just enough to figure out that the CPU usage was running at nearly 100% but only when I was using Internet Explorer. It turned out that even though I had a great virus program, I still ended up with a Trojan Horse virus.
Basically, completely remove all the programs you can. Download Firefox and use that for surfin' the net, download and run a couple of spyware programs that you can get for free, I used PCTools. BUT, delete them from your hard drive after you used them. Some spywares can actually sneak into your computer through them. My DBF's best buddy is an IT guy and he recommends using them every couple of months, but delete them when your done.
It also helps to not have any programs in your start up tray, they are constantly running in the background, and really unnecessary. If you have a desktop short cut, no need for one in your start up tray.
Hope some of this helps
