I had a dream last night where I installed Windows XP on a 286 in my elementary school and it slowed to a crawl. I had to scramble to find a windows 3.1 install disk to fix it before the teacher came over and found out.
IIRC XP needs a 486 or possibly even Pentium to run, but there were some crazy guys many years ago doing slowdown experiments and got one of those to run at 8 MHz. IIRC it took 30 minutes to boot far enough to see a mouse pointer.
We used to have a few Pentium 166 with 64 MB in the computer club, running Windows 2000 desktop version. It was reasonably OK for web browsing but not MS Office, or if it was vice versa.