Every so often, as I'm sure many do, I toss out my cyber junk and reformat the hard drive. It's gets a fresh OS install going for the year and even there may be absolutely zero improvement noticed, it makes me feel better. If anything, it is at least an attempt to scrub out any info that may be lurking around Microsoft Windows' woefully inept infrastructure regarding security and deleted files. OSX has a built-in disk utility that allows the user to do a basic, DoD 7 or DoD 35 free space wipe. Every OS should have that.
I've also recently reinstalled Ubuntu Linux 8.04 on a VMWare Fusion 1.1 virtual machine, along with the dreaded and essential Windows XP. With that XP virtual machine, I have Zone Alarm. I do what I have to online, then I lock it down. XP does not have access to my Macintosh home files and it's read only, so it's locked down pretty tight. I needed the MS to run a certain program I use, and for MS Office Excel and Word. It's hard to use computers and not use and be able to read those programs. I refuse to pollute my Mac OSX with a Windows program for Mac. Anything Microsoft is in isolation, where it creates less mischief.
Update: Thought I'd mention that a DoD 7 free space wipe took about 9 hours on a 100 GB of space. All told the reload and wipe, and downloading all of the Mac updates took the best part of 24 hours, and that's not counting favorite programs to load. If I had a faster connection, the time would be a fraction of that, but there's no fast way to secure-wipe 100 GB, not that I know of.