I ran XP for a year with a AMD K6-2 350 Mhz and 32MB of RAM. (took some coaxing to get XP on there at all). Then 3 years after that with 512MB.
It certainly wasn't what I'd call a speedy experience, but coming from my 486 it was usable for VB6 so I was happy.
And then I Installed XP onto my new PC, since I had the disk- and reverted my old one to; well, nothing, I guess. Just a benchtest machine. has Windows NT 3.51 right now, I think.