Where can I download an old version of PostgreSQL for Windows?
For good reason PostgreSQL 8.4 ceased being supported at July 2014. Moreover, PostgreSQL doesn't host compiled versions for Windows, and Enterprise DB likely doesn't want to incur the liability of hosting packages known to be insecure. Even AS-IS it becomes a question of reputation.

Upgrade your software 8.4 was released on July 2009. It's older than Windows 7.

If you must have 8.4 feel free to download the source and compile or to seek out a consultant,


  PostgreSQL for Windows can be built using MinGW, a Unix-like build environment for Microsoft operating systems, or using Microsoft's Visual C++ compiler suite. The MinGW build variant uses the normal build system described in this chapter; the Visual C++ build works completely differently and is described in Chapter 16. It is a fully native build and uses no additional software like MinGW.


You can find the source for 8.4 on git, or in a zip

Archives

As others have pointed out,


  If you are adventurous: http://oldapps.com/de/postgresql.php or http://oldversion.com/windows/postgresql – a_horse_with_no_name 30 mins ago 


However, whether or not you want to trust oldapps.com or oldversion.com is another question. I certainly wouldn't.

Linux

You could always use Linux to bring yourself up to a recent version and then dump it and load it in a version supported by Windows and hosted by EnterpriseDB.