Access Windows file share over the internet

You need some sort of VPN.

Windows 2008 server has the built in Routing and Remote Access service which has a VPN component and would allow remote users to dial in, or if you've got lots of remote users on the same network a site to site VPN might be easier to manage. This could be achieved (again) with RRAS or you could use a dedicated hardware VPN concentrator like a Cisco ASA or Juniper SSG device.