The premise

I've been tasked with finding a solution to the following scenario:

• our main file server is a Linux machine.
• on the LAN, users simply access the files using SMB.
• each user has an account on the file server and his/her own access rights.
• user accounts are simple passwd/group security accounts, not NIS/LDAP.

The problem

We want to give users (or at least some of them, say if they belong to a particular group) the ability to access the files from the Internet while travelling.

Ideally

• I'd like a seamless solution. Maybe something that allows the user to access a mapped drive would be ideal.
• A web-oriented solution is also good but it should present files in a way that is familiar to users, in an explorer-like fashion for instance.
• Security is a must of course, and users would be expected to log-in.
  The connection to the server should also be encrypted.

Anyone has some pointers to neat solutions?
Any experiences?

Edit
The client machines are Windows only.