I recently got a hand on two "computosaurs". Both are PC compatible, running at respectively 200 and 400 MHz. As I started my digital life with a 4 Mhz processor I'd tend to say 200 MHz PC are not completely obsolete yet... But for a day to day usage it seems they are!
Which OS can run on them? Windows is clearly out -- didn't try (but don't own either) Windows CE. In the Linux world, even the lightweight Lubuntu seems to be too much for these machines.
I gave Puppy Arcade a try. It works well on the 400 MHz computer, but clearly suffers on the 200 MHz one.
I should give Slitaz another look. I've the feeling it could work correctly on both machines. Stay tuned!
source: Puppy Arcade
source: What's the best lightweight Linux distro?
P.S. I had to use the USB bootable version of Puppy Arcade on both PC, as they didn't have any other removable media available.