My old, er, current pc is a 400 mhz pc.
I run win98. MS says that 233mhz is the absolute minimum (300mhz the
recommended minimum) processor speed to run winXP.
I've never seen anyone (non-MS) recommend WinXP for my pc's speed.
You may want to check the MS site to see if your pc meets minimum requirements:
http://www.microsoft.com/windowsxp/home/upgrading/default.mspx
(and looking for drivers for your old hardware could be a problem, too)
If you search google for "minimum requirements windows xp" in the *windows*
newsgroup, you'll see some real life recommendations.
http://google.com/groups?q=minimum+requirements+windows+xp+group:*windows*
(I just looked and a couple people said mine would work, but would be
slowwwwwww.)