Windows 2000 on the K7-600 machine (see installation thread). This is an OS from 1999 running on hardware from that same year. And, if you ask me, this was the best Windows release of all times: super-clean UI on an NT core, carrying all of the features you would want around performance and stability (except with terrible boot times). As you can see, things still fare very well for the old machine in terms of UI responsiveness.