There are no "Mac Viruses" at this point... There is the "annoyance ware" of MacDefender... however if you have 10.6.8 installed, then it auto-scans for it and won't let you open it. (and that automatically updates every day checking for new malware threats.).
Honestly, with talks of beachballing, crashing, etc... Your HD could be failing. I'd scan it with something like TechTool Pro 6 (Or 5 would work fine... doing a surface scan) and see if you have any bad blocks. As someone mentioned, it could also be bad RAM.
Most of those 'clean my mac' programs are pretty useless... yes they clean up some room... (get rid of cache files, log files, etc)... But really, we have 1TB HD's now... "Whoopie, I got rid of 5 MB of log files! I can save ONE more photo in iPhoto!"
_ALSO_, one thing to keep of note, some of those apps delete extra languages out of apps to save space... while that sounds like a great idea... Some apps (I know Adobe Creative Suite was one of them before) would sense the apps are corrupted, and fail to work. THEN you would have to reinstall CS. (and any of you that have installed Adobe CS, know how much fun that can be...) So well, an hour or 2 downtime for trying to save a bit of space.
HOWEVER, I do have to say this. Kudos to you for having a Time Machine Backup. I hate it when someone brings in a computer and we turn it on, and the first sound I hear is "click click click" of the HD. (Which means there is about a snowballs chance in hell of recovery) unless you want to pay $1,000 to send it to drivesavers. And I ask "Do you have a backup?" and most of the time the answer is no... Soon you'll be able to re-download any music from the iTunes store that you've purchased... But Pictures aren't replaceable people. Backup... Backup... Backup... ;)
The one app up there that IS really good is drive genius. We've had drives that TechTool Pro / DiskWarrior couldn't do anything with that Drive Genius got working again. (Catalog / File System errors).