From: http://support.microsoft.com/kb/277867

Permissions are cumulative

NTFS file permissions take priority over NTFS folder permissions.

Deny overrides all

By default, permissions that you assign to the parent folder are inherited by and propagated to the subfolders and files contained in the parent folder.

Excel

Excel updates a document by saving a temporary version, deleting the original, and then renaming the temporary file. If you can't delete, you can't update.