Set up limited folder permissions on Win XP Pro box to user connecting to workgroup thru firewall VPN box

Hello Experts,

Here is the situation: A Windows network of three Win XP Pro computers.
Set up as a workgroup - not a domain.
One computer holds the data that the others share all the time with maps to the shared folders. We'll call this the "file server" even thou it is a Win XP pro box.
The office makes use of a small firewall box with built in VPN capability. the VPN is set up and working on the firewall.
Two people work inthe office on their respective computers.
A third person, who is remote, connects from home to the firewall using the VPN connection, then uses Remote Desktop on the file server to work for hours at a time.

We want to allow a fourth person, also a remote user, to connect to the VPN and map to one (and only one) folder on the file server. We want him to be able to do this at the same time as the third employee is connected to the file server using Remote Desktop and not interfear with her work session.

1.) How do I configure the Sharing settings to allow the first two people in the office and the remote desktop user to have full access to all of the shared folders on the file server, but limit the fourth person to have access to only one designated folder? (call it the AcctData folder)

Please assume the fourth person will not need Remote Desktop.  He will only need access to one folder. He can map to it for convenience.

2.) Will we have to create a new user on the file server so user 4 can have access to the shared folder?

Any help will be appreciated!