file/pc security

We are trying to strengthen our file/pc/network security. We’d like to have some areas under maximum security. Can anyone suggest some good methods? We have a primarily NT/2000 network with a few 95/98 machines. I’d like to be able to ensure that some files can only be accessed by certain users etc. Also, is it possible to restrict users from saving files to their local machines or floppy disks? How about restricting them from saving files to certain drives? Thanks for any suggestions.