You can install any version of Linux designed to run on an intel Processor Architecture (i386 and up). The Pentium is i586 I believe, so anything short of an i686 optimized disto (Arch or JAMD) should work without any problems.
I'd recommend Vector Linux, Libranet, or Damn Small Linux at first. You could also try Mandrake or Red Hat, but you'd probably find them too slow to run without configuring them. I'm not sure how Lycoris would perform, but it might be worth a try.
If you want something a bit more challenging, Slackware Linux is great, and you learn a lot about Linux very quickly when you use it.
You will most likely want to use a lightweight desktop environment like XFce or ROX instead of KDE or GNOME. Or just use a window manager like IceWM or Fluxbox.
Read this if you don't know what I'm talking about.
