Maybe these will help you?
Or at least get you started.
AFAIK they too use tools like apt-get and the GUI "Synaptic" for that.
Xandros is (to me) a strange kind of (half?)-commercial distribution which - as already mentioned - has some limitations (because of that).
I personally would not lay my hands on this...but don't let that stop you.
You should also consult their website and/or support for how installing/upgrading software is done in Xandros.
A more flexible solution would probably be the installation of something like a special Ubuntu tailored for EeePC like jay73 already suggested.