This stuff can go pretty deep...
Right-click your C: drive and go to Properties>>Security, and then check that SYSTEM, and Administrators both have 'Full Control'. You can click 'Edit' to go to the page to change them. Windows 7 had an update a few months ago that went through and changed the permissions on the OS drive, and made a mess out of one of our servers here. We were no longer able to write to anything on the C: drive that was not concidered "User" area. If that update is on your computer, then it may have changed that permission, and after you re-start your computer it may even change the permissions back. I do not remember the update number, but I may be able to find it if this is an issue for you.