When installing VPC 2004 SP1, you might see the error "InstallScript engine on this machine is older than the version required to run this setup". While I haven't seen this error elsewhere, my research suggests other products may be affected similarly).
Searching Google, I came up with a variety of InstallShield-related links, proposing the removal/reinstall of ISScript installers 2.0, 7, 8 and 9; elsewhere I read that 10.5 is the current packaged version, under which similar errors were occurring.
This drove me more than a little nuts until I surmised it's not an InstallShield error at all, but an error associated with the Microsoft Installer. I navigated to Windows update and clicked "View Installation History". This gave me a list of updates, one of which (as of the time of this writing) "Microsoft Windows Installer 3.1", with a link that opens (after drilling down a bit) to a URL that loads a new version of Windows Installer (KB893803-x86.exe).
Don't get too excited (I did); this was not the solution to my problem after all. Running the installation program from that link updated my windows installer, but the error persisted. I tried removing and reinstalling VPC2004, but the error continued.
My solution? On a long shot, I renamed SETUP.EXE in the VPC 2004 SP1 installation share and copied the SETUP.EXE file from the original VPC2004 installation image to the VPC2004SP1 installation share. Running the old SETUP.EXE program did the trick; allowing me to install VPC2004SP1 to my system.
Caveat: VPC SP1 made all my VPC images run like mud. Once I performed the optimization and compaction tasks (which was my reasoning behind installing SP1), I uninstalled VPC SP1 and reinstalled the original bits.