Some time ago I had a stupidiest case I ever seen. One machine began crashing right after the booting.
While attaching its hard drive to a repair station (I should note we split all our hard drives into to two partitions: C:\ for the OS and D:\ for the user data) I found that Windows won't let me access the D:\ partition on that drive. The error messages were most incomprehensible, the disk checking tool just refused to work and even the disk manager just refused to open that partition's properties sheet. In short, it looked like that partition was history and needed to be re-formatted.
But I managed to figure out that it was a severe case of the NTFS permissions corruption. Something happened, and the D:\ root just lost all its permissions, as a result there was no one with the access to even list its contents! (usually, at least "SYSTEM" has it). So naturally Windows refused to work with it.
This stuff is solved via some shamanism with the ownership - there's a some workaround built into Windows itself. You must dig up the drive's advanced permissions menu (the "use simple file sharing" must be off, of course), find the ownership section... I forgot the exact details but if you dig around enough, it will ask you if you wand to change the ownership to yourself. After that is complete, the permissions controls become un-grayed and you can start adding users with rights to read and write that partition.
That said, most repair installations fail to fix the problem.
That's true. The most sensible thing is to attach your hard drive to another computer, back up all your data, and then re-format it.
That's an easiest way.