One of the annoying aspects of using NTFS is that, from time to time (eg. if Windows 8 preview corrupted your Windows 7 partition so bad that you had to reinstall the whole OS - see post #7), Windows will decide that you are no longer entitled to access your files, and while performing some operation or other you'll get something like:
There's nothing more infuriating in terms of computing than being denied access to files you should have access to.
The solution? Open an elevated command prompt and issue:
takeown /R /F <Drive or directory>
Icacls <Drive or directory> /reset /T