As few as possible:
As operating system Windows 2000, Windows XP, Windows Vista and Windows 7 are all supported and tested.
PackageShell is developed in native C++ and has no dependency on any run-time-environment or other DLLs. The whole product consists of a single executable (about 200 KB) with optional DLLs to support additional languages.
No .Net Framework or even MFC was used in development because PackageShell itself is commonly used to install or reinstall such system components.