Who Has Access?

I was just trying to backup all the data on my office computer so that I could upgrade from (cussed) Vista to Windows 7. I selected a folder on my hard drive to copy to the backup external drive and a little screen popped up that said, "You do not have access to this folder."

Excuse me?

It's my computer . . . as in, I own it. As in, I'm the only administrator with any rights whatsoever as to this computer.

Which begs the question . . . if I don't have access to that folder . . . who does? And when are they accessing it? Do they come in the middle of the night and log on, do some work, and then leave before I saunter in around 9:30?