Can I decompile an existing installer?

Currently NSIS installers cannot be fully decompiled. The installer itself doesn't provide any method to extract files or the script without installation. It is the developer's choice whether the source code and/or the files for the installer are available to the public or not.

There are, however, external tools that allow this. 7-zip (version 4.40beta at the time of this writing) supports decompressing NSIS installers compressed with lzma or bzip, but the source code is still partially compiled and requires extensive modification before the script can be recompiled. A decompression plug-in is also available for TotalCommander.

As a general note to software developers, you should use a plugin like DCryptDll if you need to protect certain files in your installer.