SONAR 2015 Clean Install document suggests deleting the following registry key before re-installing SONAR:

HKEY_CURRENT_USER\Software\Cakewalk Music Software\SONAR\<Platinum/Professional/Artist>

I have SONAR Platinum
My question is:  Do I delete the entire "...\SONAR\Platinum" folder, the entire ...\SONAR" folder or is there a specific file?  
There are a bunch of files in the "...\SONAR\Platinum" folder.  E.g. 64BitMixing (Type: REG_DWORD)
Is the folder considered a "Registry Key"?  
I'm assuming since they didn't specify a specific file name, I delete the entire folder?

The idea behind a "clean" install is to have the new installation "think" this is the first time it's ever been installed in this machine. One of the first things a software installer does is check the registry to confirm this is the case. 
 
So your best bet is to obliterate any files that are associated with Cakewalk/ Sonar. 
 
It goes without saying that all your personal project files and any 3rd party VST plug ins are all backed up to a DATA drive somewhere. 
 
Ultimately you install a new hard drive and OS, that's a squeaky clean install.