How can I password-protect an entire folder in Excel?

I want to password-protect an entire folder in Excel2003, but don't know if I can. I right-clicked on the folder, then on Sharing & Securities, then Sharing. There is an option "To make this folder and sub-folders private....." by clicking the box, but the box is gray so I can't do that. I know I can put a password on each individual file, but I was really hoping to do the folder. Thanks!!! edit