We got a clean build of XP put on PC's in our company with specs as low as 300Mhz/64B RAM.
They worked for a while until further updates caused them to die . . . horrifically !! Laugh . . .?
Yes, unfortunately Microsoft are always conservative with their minimum requirements - imagine if they put the 'real' minimum - it would warn 80% of people that they needn't buy the software as it wouldn't run on their PC's then MS wouldn't sell any new software.
Seriously, I think it would be a good topic for Speakers Corner.
My first works computer was an IBM something with b/w screen and a 360k floppy. Can't remember much else about it but I progressed (via another job) to the ubiquitous Amstrad 1512 (twin 360k floppies, no HDD) and an Amstrad 1640 with a 20MB Hard Disk which eventually failed and we had to buy a WD 30MB Hard Card !