Locking Files ??

[Poppy]

I have a major problem here.
I have an asp intranet app which allows users to download files from the
server.

It all works fine but now I have been told that in the case of Word and
Excel files the user, after downloading or opening the file the file must be
locked in some way so that other users cannot change it.
On top of that I also have to allow users to save changes they have made to
these files on the server.

Is any of this possible ?

Thanks in Advance