My last install of the Mac OS took 25 minutes tops. It took an additional 20 minutes to run software update and get all the program updates I needed. My install also didn't require an older version of the Mac OS to be on my computer. Windows Vista requires a valid XP installation (otherwise you have to pay $499 for the full version).
A local (to me) television station tech news analyst took 4+ hours to install Windows Vista and it still seemed like after he was finished it was a nightmare.
The most ironic thing about the video commentary linked above is that this was termed as "going pretty smoothly".