Reset File Permissions:

The repair will grant administrators, system, users and everyone full rights to every file on the selected drives.

This will only ADD the permissions, it will not remove current permissions or the owner of a file.

When certain files lose their proper permissions, then lots of problems can happen, such as not being able to install windows updates, program errors, services failing and so on.

WARNING: This repair can take a very significant amount of time to complete, depending on the speed of your CPU & the number of files on the drive.