I still have NT 4.0 install media around, and have even tossed it on an older box a few times to remember what it was like back in the days when Windows could be a decent developer OS.
Unfortunately, anything older than Win2k leaves you pretty much out of luck for mobile computing (power management, WiFi, and display switching) as well as most modern peripherals (USB support being notably lacking).
For even more retro fun, though, I recommend firing up a copy of Mac OS System 7 inside Basilisk II or SheepShaver -- even under emulation, it absolutely screams through day-to-day tasks.