Here are a couple of extra things you can try, if you open a command line (Start / Run / "cmd"):
DIR
note the free space left - I wouldn't want to see it go below 200MB or so
CLEANMGR
this is the "Disk Cleanup" program that looks for temporary files etc. In this mode, it will stop and tell you what is using space, such as "Temporary Internet Files"
If you have time to spare, try this:
CHKDSK C: /F
it will say "can't examine the disk now, shall I do it at next startup?" If you say "Y", it will run a full disk check at next boot, which may take some time, depending on how big the disk is.
I haven't used Cakewalk recently, but I do remember their
Audio Finder Tool. That page doesn't say it works with Sonar 6 - but it might.