I have stayed out of this thread up to now. I was not prepared to get into the Apple verse PC, OS X verse XP or even you have a virus or not.
The bottom line you have a problem which you are looking for a solution. Since you have decided to use your system as an expensive paper weight. Your next logical step is to do a clean install of the OS.
Since you have been around the block a few time I assume you know how to do that.
Also I know that witeshark gets carried away with repairing permissions, I assume that he make frequent changes to his system that warrant, his actions. I only repair the permissions after I install or update software.
As for the cron jobs (daily, weekly, and monthly) he mentions, these jobs are to clean up all log files. if you don't roll them over from time to time, they can get quite large and eat a massive amount of disk space.
So getting back on point, I was I had a real solution to your problem. I know it pains you. All I can suggest is starting with a fresh OS X load then, work with your mail. This sould rule out any other influences, third party software configurations.