I have used DSL on PII 300 Mhz no problem. If it has 128 Mb or RAM, you can load full OS to RAM and is very fast even on old machine.

I also agree with previous post as to not running resource intensive applications. I have run Knoppix 3.9 on a PIII 400 Mhz nicely with Fluxbox instead of KDE. Give Fluxbox a try, right-click to get the menu tree. Use cheat code "boot: desktop=fluxbox" to start it at boot.