Actually, Microsoft are quite right here:
http://www.microsoft.com/windowsxp/sp2/sysreqs.mspx
I just installed XP SP2 on a laptop from 2000, Pentium 3 - 400MHZ and 128MB Ram.
Works quite fine, not fast, but good enough for my mother 
I tried with a 233MHZ, but that doesn´t work well, Windows 2000 would be a better choice.
