What is the easiest way to share documents among a small office (9 people)
that forces check-out/check-in? We really need to be sure that only one
person at a time is editing each document.

Revision history and backups would be nice, but the biggest thing is
ease-of-use.

I'm not sure of this is something that you can do natively in Windows or if
you need a special product to get this capability.

If it has to be a product or add-on, something that integrated into Excel,
Word and Windows Explorer would be great.

Thanks!