See the video, which elapses in under 10 minutes, but took three full days to record.
Check out Andy's blog post explanation of what he did.
The whole procedure took 3 days, of which approximately 12 hours was spent doing the actual installing - the rest of the time was spent trying to obtain valid copies of windows.
Another hurdle that had to be overcome was one of system requirements. MS-DOS uses the FAT16 filesystem, which only supports partitions up to 2GB in size. This was no problem until Windows XP, which required a comparatively large amount of space - at this point, the filesystem was converted to FAT32.
I was pleasantly surprised to see the DOS versions of Doom 2 and Monkey Island still running in Windows 7 after all these years. Microsoft has had a lot of stick through the years (not least from me) but this really is a laudable achievement.