At a bare minimum, you'll need the following for both the buildmaster and a buildslave:
Buildbot requires python-2.4 or later.
Both the buildmaster and the buildslaves require Twisted-8.0.x or later. As always, the most recent version is recommended.
Twisted is delivered as a collection of subpackages. You'll need at least "Twisted" (the core package), and you'll also want TwistedMail, TwistedWeb, and TwistedWords (for sending email, serving a web status page, and delivering build status via IRC, respectively). You might also want TwistedConch (for the encrypted Manhole debug port). Note that Twisted requires ZopeInterface to be installed as well.
Of course, your project's build process will impose additional requirements on the buildslaves. These hosts must have all the tools necessary to compile and test your project's source code.
Buildbot - both master and slave - runs well natively on Windows. The slave runs well on Cygwin, but because of problems with SQLite on Cygwin, the master does not.
Buildbot's windows testing is limited to the most recent Twisted and Python versions. For best results, use the most recent available versions of these libraries on Windows.
Note that versions of MSys Git earlier than 220.127.116.11 are known to have problems with incorrect exit statuses from git commands. Always use the newest version of MSys Git.