What is the error message? Is is coming from ISA, or is it a IIS error about not reaching a destination due to an ISA config?
Basically, one or the other is giving you an error, and it has a mapping to an .html, .htm, .asp, .php etc file that gives you the error. You could literally look up the file name using the search on the ISA machine to find it. Either way, it is most likely in the Inetpubwwwroot folder.