Are you logged on to your PC under an Administrative Account or an account that has Admin. privileges?
If you are, and can't access regedit it could be a problem with registry permissions. I had a similar problem where I wasn't allowed into certain files even though I am the Administrator.
Due to several reasons, permissions can get all out of whack.
Download "Dial-a-fix" v0.57.7 - can be found here: http://wiki.djlizard.net/Dial-a-fix
If you are using XP download the medium version.
Run the "Repair Permissions" feature under "tools".
It takes a little while to complete it's mission.
I found it to work well for me. I was able to access files I couldn't before, and some programs I was having trouble with worked like they should.
Hope it works for you.