Not to make the unpopular recommendation or anything... But I have had decent success with Win2k Pro on my Micron Transport XPe... Intel 150MHz MMX, 80MB RAM. Win98SE will also work well.
It's the amount of memory that hinders the original poster's ability to run NT based Windowsen. For win2k you need at least 128MB RAM for it to be nice to use.. The more the better.
With today's RAM prices it's often a good idea to buy as much RAM as you can fit in that old PC. It may seem like overkill to have 512MB RAM, but you'll get more out of that old box if it doesn't have to swap to it's slow disk all the time.