I would like to set up a internet file server to use in a satellite office?

We currently have a computer that acts as a file server for a small office of 5 employees. We are considering getting a retreat that we would like to work out of a couple times a month. If we use windows xp on the current file server, what would be one of the best options to make those files accessible over the internet so we can work on them at a different location? Currently everything is real time and a saved file can be opened while someone else is working on it and we can see the changes they make instantly every time they press save. This is for autocad, word and excel files.