Uninstalling, using CCleaner to scour the Registry, deleting everything remaining, and reinstalling, as time consuming as all of that was, the results have been TERRIFIC. I had ArmA 2 CO installed a year ago, and the difference in performance is incredible. My framerates with blur on were awful back then, but now, my framerates are MUCH smoother. And I'm still using the hardware I had back then.
So, unless this turns out to be a virus (and Norton came back negative on that), I'll consider this degradation of the game a good thing, in the long run.
Just to be safe though, I backed up the entire game on my external hard drive, without any mods being installed, and without me even starting it up. It's as pristine a backup as I can imagine. And with my firewire connection, far faster than reinstalling from the discs.
Thanks for the advice!