This issue can occur from not having a required Microsoft update prior to installing Act! on a 64-bit edition of Windows®. If the version of Window installed is Windows Vista®, the error message will refer to Microsoft Update KB929916. If the version is Windows Server® 2008, the error refers to Microsoft Update KB950760: To obtain instructions to acquire these updates for your installed 64-bit edition of Windows, please refer to the following Microsoft Knowledge Base documents:
Windows Vista® 64-bit
.NET Framework 2.0 SYSTEM.WEB.DLL Security Update for Windows Vista® (KB929916)
Windows Server® 2008 64-bit
Cumulative Security Update for ActiveX Killbits for Windows Server® 2008 x64 Edition (KB950760)
After applying the correct update, reboot the computer and then proceed to install Act!.