Microsoft said that the minimum requirement to run Windows XP are CPU with 233 MHz an 64 MB of RAM, but some people in Germany had tried to beat that official requirement by trying to install and run Windows XP in the most minimum specification and they made it even though it's very time consuming (but they deserve to receive the The golden Sandclock Award For extreme waste of time for their efforts).
After five efforts, they can install Windows XP and finally reduce the spec until they can reach 7 MHz and 20 MB of RAM. The boot time took about 30 minutes (which is normal under that kind of specs), but everything is working just fine (Of course the CPU is at 100% all the time, there is no real idle mode). I guess Microsoft had to revise their official minimum specification to run Windows XP after they see this experiments :D
Here's the screenshot of the most minimum specs: