ThinkPad T42 doesnt have support for PAE so if you want to install a distro make sure you download the non PAE version.
A good distribution for you laptop would be Bodhi Linux which offers a 32 bit Non PAE version
Here is the link http://www.bodhilinux.com/downloads_desktop.php
I dont know if Lubuntu offers a non PAE option.
Another OSs I had good succes installing was Arch Linux, SlackWare Linux, and FreeBSD 10.0
Those are good but kind of crazy installing them, but they do work on your Thinkpad