Windows Installer always uses msi "databases" (or msi files) for installation. the exe based installers that "integrate" with Windows Installer are simple wrappers for the msi databases, the msi databases themselves are either stored externally or are embedded in the exe file itself and are extracted, in this case it's the latter. I used NSIS (Nullsoft Scriptable Install System) to create the wrapper and WiX (Windows Installer XML) to create the msi database itself. In NSIS I didn't use the Registry or an external command for installing the program or detecting whether the program is installed, instead I used the Windows Installer API. Microsoft published documentation for it  here. The only problem I had with their documentation is I could only find the name of some of the constants I needed to use, but not the value, which is a problem since those constants aren't defined in NSIS. Fortunately I didn't have to reverse engineer anything to figure them out since the Wine devs already did this for Wine's implementation of Windows Installer, I just had to open up the source code of Wine's Implementation of Windows Installer and got the information from that. Although I could probably get the same information from the Windows SDK but I didn't want to bother downloading it since the headers in Wine are the same as the headers included in the Windows SDK, compatibility and functionality wise. NSIS can call external dlls using the System Plugin which was how I was able to use the Windows Install API with NSIS. The buttons that asks whether you want to repair, remove, add/remove features that a lot of Windows Installer programs have, I used the Windows Installer API to detect whether the program is already installed so it knows whether to show these buttons show up or not. To get the buttons to show up I used nsDialogs information about nsDialogs can be found here.