Help! my Sonar files are a mess.

First off I bought in at 8.0 something then upgraded to x1 studio, then to x1 pro, then expanded.
All those original files and programs are still there taking up space and it's really hard to know what to get rid of, so I kept them in case I trashed something important.
Then I was told to keep projects on another hard drive.
Then that drive started to fail so I bought a new one and moved the files back to the C drive
Next I have been recording various versions of songs (unplugged, rock, full orchestrated etc)
Then I started keeping saved files (projects) in the "Audio Library" but not all are there either and not (necessarily) under "Projects".
I finally see that a more systematic approach would be best so it would be great if I could clean up the files and have all projects in one folder.
I do (now) save the Audio files with the Song folder. This is better, but there are still the old ones to deal with.
It would be nice to stop opening "Cakewalk", Cakewalk Content" or various "Sonar" folders and finding videos, or install files or empty Project or Audio folders scattered about.
I started moving files about in Windows Explorer (the horror) but now sonar can't find files associated.
It's a mess and I've gone so far now over time that I can't remember what I did when and where in order to undo.
Yes I know I'm an idiot for thinking I could move a song folder from one place to another without screwing it all up. I've never had a program that broke up it's file structure so completely and confusingly.

What files are needed to open a project? (Just the song folder with it's Audio folder inside?)
What files should be kept on a separate drive?
At least I don't do videos.

Thanks FBB
I have been doing the Per Project for a while now and it's working great.
It's all about older projects. 
IOW Before I started the per project method. 
I need to open them in order to do the Save as idea.
I guess I need to know what do I look for when Sonar says "Files not found" and then leave it blank.

Thanks
there seem to be two conventions.
One is as I said, the other searches for files but can't find them and replaces them with silence.
There must be a huge file or two somewhere that has them all and I'm sure I didn't erase them (The problem is more about having duplicates and unnecessary files) but what folder would I look for?

Aha, that helps I think.
That was set to an empty folder.
Changing it to the current root folder was the answer
Thanks