OWA2000 behind Firewall for Exch2000

Hello All,
I have an ISA Server with two NICs(one for LAN and other for Internet-with own live IP address), being used as an ‘Entrance’ to my Windows n/w. I have Win2K Server with Exchange2K, which is not ‘live’ on internet; mails communicate via the ISA to & fro. I would like to access my mails that resides on my Exchange2K from outside LAN – in a secure manner. How do I go about this with simplicity and less modifications?