To be honest I am not familiar with ISA2000 so I don't know how to disable FBA using it.
There is a guide on publishing Exchange 2003 with ISA 2000 here:
http://www.msexchange.org/tutorials/pubowa2003part1.htmlBut for all intense and purposes if your not using SSL all you need to do is forward port 80 to your Exchange server and then turn on FBA as detailed above without SSL on the Exchange Server.
So you would just publish it as a normal website.