Snow Leopard Memory Usage

Question

I have recently downgraded my iMac 8,1 back to Snow Leopard due to an unsupported video card in Lion. Not soon after I downgraded, I started experiencing high cpu usage when the machine would run for long periods of time without being rebooted (6-7 hours).
Currently, the machine has 8GB of RAM (which is all recognized in spite of the documentation stating the max ram on this machine is 6GB). I run, on average 8-10 applications at the same time throughout the day. It appears that the offending application that takes up cpu usage varies throughout the day. This morning, it is iTunes using 85%. Other times, it's Apple Mail. Even others, it's MDWorker (which I'm not confident I know what that is).
What doesn't make sense is that the offending application changes throughout the day. As time passes, sometimes when I attempt to launch a video on a website, it gets real choppy and will not run normally until I reboot. The audio cuts in and out and the frame rate is horrible.
My initial thought was that one of the memory sticks is bad, but these issues don't indicate a memory problem from what I understand. Yesterday, I removed one of the two sticks of RAM and booted to see what performance was like with just 4GB in. It did the same thing even after only 5 minutes running. I have also considered a memory leak, but that in itself shouldn't cause abnormal CPU usage.
Currently, My CPU usage is at about 50% on both cores with iTunes taking up 82% (I am streaming audio) and the Activity Monitor itself using 8%. All other processes are under 2%.
Does anyone have any advice? What should be normal CPU usage on Snow Leopard? Of course, that's a subjective question that depends on what applicatios you are running. I do not do any heavy graphics processing. My main applications are Safari, Word, Excel, Outlook, iChat, and iTunes. I am thinking of, once again, nuking this machine and rebuilding from scratch.
Any advice would be helpful. I'm pulling my hair out.

Answer 1

RevJoel wrote:
On a hunch, I ran ClamXAV from Safe Mode and found 8 emails with Trojans. While not critical, I moved them to the trash and emptied it. One of the 8 was in my Mail folder. The rest were in SPAM. I rebooted after that was complete and have been running better since the above screenshot was taken. I cannot imagine that a trojan was clicked on as it was probably an attachment unless someone I regularly email with accidentally sent me one since that's how they work.
There are no known Trojans spread through e-mail that affect the Mac and without knowing the infection name I can't really say, but suspect they were all Windows. Most Mac malware detected by ClamXav have "OSX" as part of the infection name.
It's never a good idea to move e-mail files around using the finder or any AV software. This will most certainly corrupt the mailbox index file which could result in the loss of additional e-mail. You should repair the damage by running "Rebuild" from the Mailbox menu of Mail on both your Inbox and Junk folders. It's also possible that you didn't delete the oriinal mail from your e-mail ISP's server and it will just be downloaded to your computer again the next time you check for mail. Depends several settins in Mail and on your ISP's server.
You should always use the "Reveal in Finder" option to locate the e-mail in the finder, then double-click it to open it in Mail, then use Mail's delete key (and empty the trash folder if you use it). There may be  one more step to permanently delete it if the e-mail is from a gmail account.