I believe the Scratch Windows-Installer is a compiled NSIS script:
http://nsis.sourceforge.net/Main_Page
The Scratch Team has not (yet?) released the actual installer source code. Probably not, however, because there would be any secrets in there. You might as well download the files-only version and extract it to a folder of your preference, as Scratch doesn't need any fancy system-modifications or external packages to run. The only two things the installer *does* take care of are:
1. file associations for *.sb and *.sprite files in the windows registry
2. shortcuts
As already mentioned, you don't need any of these, Scratch will run perfectly fine without changing the windows registry (and you can do the same using regedit).
Same with shortcuts, you can create these yourself easily.
To sum it up: You don't need the installer, you might as well use the files-only version for deployment in a network. You can even run Scratch from portable media such as memory sticks without any installation at all (isn't that something!).
Please let me know if this answers your question...
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