I have spent many hours already removing registry keys, shutting down Act services, removing Act from the startup process because the Uninstall did not work on my machine. I have confirmed that I am doing everything under the Administrator privelege level.
I get the following from the main install:
1607: Unable to install InstallShield Scripting Runtime
I get the following from the ISSscript8 install:
Error 1721: There is a problem with Windows Installer package. A program required for this install to complete could not be run. Contact your support personnel or package vendor.
What I don't understand is that Quickbooks Pro 2007 installed perfectly fine on this machine. It seems like there is residual Act info on the machine but I have been on a multi-hour hunt and destroy for all the pieces that need to be removed as described on the Sage site.
I'm getting exactly the same error when attempting to install. I've found the guidance in the ACT! knowledgebase but get the same error that you mention when trying to install the Windows Installer from the ACT! CD. Anyone else encountered this and willing to help out? I've been working on this for a week and don't know how to fix this problem.