Flagged as off-topic. Maybe a better place to post this would be at http://unix.stackexchange.com/? AskUbuntu is for ubuntu-related questions (and this seems more like a general linux-related one).
– MiJynJun 18 '13 at 20:32
Can you qualify what a "very old computer" is? Specs? CPU? RAM etc? Generally speaking, you can't go wrong with Lubuntu because the desktop environment is super-lightweight (it runs on a 12 year old laptop I have) but there might be a better option depending on your exact specifications
– KentGeekJun 18 '13 at 20:33
I have a PIII, with 768RAM, and I use WATTOS. Its Ubuntu based. Try it. R7 is based on Ubuntu 13.04, and R6 is based on Ubuntu 12.04. I went with R6, much faster.