From NSIS Wiki
NSIS (Nullsoft Scriptable Install System) is a professional open source system to create Windows installers. It is designed to be as small and flexible as possible and is therefore very suitable for internet distribution.
Being a user's first experience with your product, a stable and reliable installer is an important component of succesful software. With NSIS you can create such installers that are capable of doing everything that is needed to setup your software.
NSIS is script-based and allows you to create the logic to handle even the most complex installation tasks. Many plug-ins and scripts are already available: you can create web installers, communicate with Windows and other software components, install or update shared components and more.