Honestly, you won't get much mileage out of a 100mhz machine but if you...

Honestly, you won't get much mileage out of a 100mhz machine but if you really must, then slackware is what you want :
http://www.slackware.com/
You could try a floppy install using a network card :
http://www.slackware.com/install/rootdisk.php

If the latest version of slackware doesn't work for you, version slackware-10.2 should. It's an older version and I seem to remember it was the only thing I could install on a 100mhz Pentium Pro years back.
ftp://ftp.mirrorservice.org/sites/ftp.slackware.com/pub/slackware

Click Here!