I tried to find out lightest distribution to IBM ThinkPad 600 (266-400MHz, 160Mb RAM), and after concluded that Xubuntu is too slow, I was pleased to speed what Puppy Linux gived, it has all relevant and up-to-date programs, all tweaked down to be as light as possible. http://www.puppylinux.org/
define "old software" . i disagree with claim that dsl has old software.
For example OpenOffice.Org was a bit older ... I don't remember was it even 1.5, but at least not 2.2
Anyway, there was more recent versions of software in puppy linux.
Also, it was very easy to setup IBM ThinkPad.
It noticed that it was that buggy 600 series laptop, and offred to setup cludges to sound and power save modes (APM). Didn't needed two months of browsing forums and trying various tweaks to accomplish that.