You gonna need a lightweight distro, or if you know how a custom install of your favorite distro, If you dont know much about Linux and only know how to install in default way, you should avoid distros with DE GUI, like KDE, Gnome, and such those are fat and heavy, and will stress your hardware to death. Now WMs are light and fast to have in old machines. Here
you can find all about DEs and WMs.
About the distros that are lightweight good choices are Debian
(custom install), Slitaz
(no so light).