Stackoverflow, Super User, Serverfault, and meta.stackoverflow are part of the Stackoverflow trilogy, and are run on one NON open source codebase.
That codebase was licensed to Fog Creek software, who are converting it into a platform for running hosted sites based on this architecture, which you can use to make your own site right now at http://www.stackexchange.com - it's free during the beta, and for 45 days thereafter (beta is still open, no official close date). So try it out.
The codebase is not available anywhere else.
As mgb noted, there are several other open sourceimplementations of the stackoverflow Q&A concept, but none are quite as complete as either stackoverflow or stackexchange. The only instructions for installing any one of those sites would be with the maintainer(s) of the respective projects.
As the original code base is not open source, stackoverflow and fog creek are not able to support any installation or usage issues with other software.