Windows XP On 8-MHz Pentium With 20 MB RAM
 

The guys over at winhistory.de managed to get their Windows XP Professional running on a very minimal box: an Intel Pentium clocked down to 8 MHz with 20 MB of RAM. (The installer won't work with less than 64 MB, but after installing you can remove memory.) (src: Slashdot)

http://www.winhistory.de/more/386/xpmini_eng.htm