Your best bet will be to setup VPN from each of the remote employees to the main person’s home. This way they can access the files as if they were on the same network.
Cisco has some inexpensive routers which can handle the VPN automatically. There are also software solutions available.
GoToMyPC or LogMeIn have subscription type solutions for this. This might be simpler than installing and managing the hardware by himself. Note that Windows XP Pro is not really meant to be a fileserver for hosting files or services. It is meant to be a single-user operating system.