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Application Installation
Lack of support for 16-bit DOS, Windows®, or OS/2 applications on 64-bit Windows presents a significant barrier to application installation programs, because they often use a 16-bit stub to check the machine type, then launch a 32-bit install engine to actually perform the install.
To work around this problem, 64-bit Windows recognizes specific 16-bit installer programs and substitutes a ported 32-bit version. The list of substitutions is stored in the registry under the following key: HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\NtVdm64. There is built-in support for several installer engines, including InstallShield 5.x installers.
Note that the 64-bit Windows Installer can seamlessly install 32-bit MSI-based applications on 64-bit Windows.