It might be because you’re trying to install it on a trial version of the ISA firewall.
Check out this thread on the ISAserver.org message boards:
http://forums.isaserver.org/m_2002068660/mpage_2/key_/tm.htm#2002069981
HTH,
Tom
Thomas W Shinder, M.D.
Site: http://www.isaserver.org/
Blog: http://blogs.isaserver.org/shinder/
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MVP — Microsoft Firewalls (ISA)