I don't know if it should be considered cheating to use an emulator because they have a very different CPU instruction timing profile than real hardware; here's a notable result from before, with real hardware running XP down at 8MHz:
http://winhistory.de/more/386/xpmini_eng.htm
Also, it looks like he started with something resembling WinPE, which is already stripped-down compared to a regular install.