RetDec is a retargetable machine-code decompiler based on LLVM
...Detection and reconstruction of C++ class hierarchies (RTTI, vtables). Signature-based removal of statically linked library code. Static analysis of executable files with detailed information. We currently support Windows (7 or later), Linux, macOS, and (experimentally) FreeBSD. An installed version of RetDec requires approximately 5 to 6 GB of free disk space.
Decompiler for setups creates with the Nullsoft Installer
Python scripts that'll help you to nearly fully recover your NSIS-installation scripts.
It's currently in the 'alpha-state' however it's not far from being worth to title as beta or release.
I'm new to Sourceforge - for small updates I may also prefer the page below.
UPDATE: It'll maybe not working on this anymore. http://7-zip.de does a great job in also decompiling NSI.So I recommand you to check out 7-zip BETA.
Generally 7-zip opens NSI[exe] files - since 9.33 [2014-06-15]...