there is not enough RAM for Kubuntu (as others already said) but there are a few linux distributons (some Ubuntu based) that will work on your mashcine. i have something similar with slightly weaker CPU.
Some good and relatively easy options that are Ubuntu based are:
Puppy Linux Precise (though not really ubuntu based, the precise version is made to use Ubuntu repositoies).
Chrunchbang - this one is Debain ("ubutnu's father") based but is still relatively easy to install. It runs a nice script after basic install so oyu can just add what you want and say no to the rest.
there are a few more dedicated to older hardware:
Tiny Core linux
DSL (damn small linux) - this one evenuses very old kernel
i would start with lubuntu but you won't be able to use graphics install. so just use mini.iso and then add lubuntu desktop package to it.
on a similar setup to yours i use Chrunchbang (but an older version that came with xfce). it takes about 80 MB ram on boot. i can write, watch you tube videos, play a few older games... nothing much. it acts as a netbook. it's fast enough for what it does.