If the machine has 128MB or more of RAM Xp will work just fine on that machine, though it will take awhile to boot.
Just don’t expect to do much more than word processing/spreadsheets or simple server apps on it.
Win2k would be your best bet due to memory usage I had it running on my old celeron 300 server with 256MB or ram for the longest time. Running two instances of a custom game server app. And a rousing game of Transport Tycoon. Worked quite well.
Where as my dual p3-550 setup took about 7 minutes to boot, but ran fine upon booting.