ISA Server 2000 Error Preventing SP2 Installation

Today I was installing ISA Server SP2 on an ISA Server 2000 computer and ran into this roadblock where it simply wouldn’t install the SP2. Here’s what the computer configuration looked like.

Windows 2000 Advanced Server with SP4

ISA Server 2000 with SP1

No IIS or SMTP service running on the server
When I tried to upgrade SP1 to SP2, it copied a whole bunch of files and then I saw the following error.

I clicked OK to uninstall partially installed SP2 and then started trying different techniques. At one point when I installed SP2 and received the exact same error, I carefully clicked OK because clicking Cancel would have left it in a state of confusion. Unfortunately, even after clicking OK it came up with the error that ISA Server is now in a partially updated state and may not work correctly.

I tried to remove SP2 using Add/Remove Programs but it said I’ve already installed a later hotfix so I can’t uninstall SP2. Basically I was stuck. Some of Microsoft’s KB articles indicated that the file FLTRSNK1.DLL seems to be tied to inetinfo.exe so I decided to install IIS and SMTP service and then tried to reinstall SP2. At that point all 5 ISA-related services and RRAS were stopped. That did the trick. SP2 finally installed successfully. After rebooting the computer, I removed IIS and SMTP because this server didn’t need those services. Everything seems to be working fine at this point.