this is very simple to do first configure two server with window server operating system and three NIC cards.Connect on NIC to both server which work as a heartbeat to sense the stability of one another.Install ISA as a CSS on Ist server and you do the same on second server but when it ask for CSS server just add the Ist server name and it will start installing.
After full installation you have to restart the server and when they comeup both work up in NLB mode.
