windows xp MC folder security

[sinnedam]

I was playing around with windows media center the other day, and noticed that if I right click a folder and go to the properties, under security there is an checkbox that says something to the effect of (sorry im not at my computer right now im at work) check this to deny access to certain users to this folder and sub folders..thats what I want to do, but for some reason the box is greyed out and I cant check it. I am running NTFS, and this is a stand alone machine, no domain.

Thanks,

D.