WinHistory.de has recently concluded an experiment wherein they tested the lowest configuration PC Windows XP could support.

The result:

“The setup worked without any problems, you only had to spend more time. Windows found a Pentium, but it does not matter if it is a Pentium I, II, III or IV. The 64 MB of RAM are at the setup MINIMUM, if you have less at this point you get a error and the setup will be ended before it really started. The installation or starting of XP on a 486-system is NOT possible.
After the setup XP is not as fastidiously as before, you can easily remove memory.With 16 MB XP will not start anymore, you have to have a little more,”

Read the full experiment here