You could try the free proggie MSICU.EXE. This undoes installer issues with MSI (you can zap the entry for the required proggie to make it look uninstalled. Google finds it fast.
Just checked the MSICU.EXE wont help your installer is Nullsoft (NSIS) you could try silent install see what happens it may overwrite not sure Code: TheGreenBow_VPN_Client.exe /S Or even use system restore and put system back to before you installed it.
Personally i use perfect uninstaller. It works very well, getting rid of all traces of an uninstalled program. Very handy if you are evaluating software and the trial period expires.