Search Microsoft's Knowledge base for "regedit has been disabled by administrator"
There is an article on there about how to re-enable that, along with descriptions as to how it may have become disabled without your knowledge. I found this a couple of years ago when I had the same problem.
As to not being able to find the C: directory, that's a bigger, and in my opinion, a more serious problem, as that could mean that your hard drive is failing.
