Backup strategy...

I use windows server 2003 for a file server.
I wan't to make a backup scheme on Mon normal backup, and tue-fri differential append. Then after a month another tape and so on.
Tell me what happens, do I need bks files for another tape.
Tell me also, when a user asks for an excel file that he owerwritten in say monday of the second week, and wants a copy of the file of tuesday of the first week. How can I restore from and append file, how can I choose the date. Because I have fot 5 files, 1 normal backup, and 4 diff. I know that first goes normal, and the latest diff. But what if the backup has only saved version.

Thanks in advance.