Extract the contents of the original exe either by starting the install and copying the files out of the %temp% folder or by using something like "UniExtract"
Note: I don't know if this step is really necessary but i had already extracted mine and it worked.
run "setup.exe -r" and run through the full install
in %windir% a file called setup.iss will be created.
copy this to the same folder as setup.exe