Just a quick note – if you’re planning on upgrading Windows Server 2003 to Service Pack 1, and you have ISA 2004 Standard Edition already installed, you should install ISA 2004 SE Service Pack 1 first – it copes with a few of the changes in WS2003SP1 better than the original release.
ISA 2004 Enterprise Edition is equivalent to SE SP1, so shouldn’t need fiddling.