Sonar never completely uninstalls just like most software. It leave a few folders behind. You have to manually delete them.
And what I do with my plug ins is copy paste the dll's to the main Cakewalk plug in folder. I have a few third party and then some from Cubase and Wavelab. I leave the originals alone and just copy/paste them. Seems to work better then the scanning thing.
I have a folder on my external back up drive with every plug in I have ever stumbled across. Some seem to install as software, but most are just the dll files and are easy to work with.