Great and I thought this would be difficult!
Right, all you need to do is custom install any distro you want and only use either XFCE or LXDE and you should be set. Easy as pie.
For a newbie I would go with Ubuntu and change it over to LXDE or OpenBox since that would be lightweight enough for your machine. XFCE might work but seeing that it uses Gnome libs underneath and Ubuntu tweaks the heck out of everything, I would be cautious unless you have time to experiment.
If you are proficient then just install Arch and use any of the lightweight WM's, Fluxbox on Arch would be my poison.
I know there will be people touting PuppyLinux and DSL and even Knoppix, but I would rather use a distro I love and tweak it till it worked like I wanted it to. But in the interest of being complete, if you need a "once-size-fits-all" solution then check out PuppyLinux, DSL and if you are up to it, a customized Knoppix (Debian).