Wubi/Ubuntu installer

I have a Netbook which has 1GB RAM and a total HDisk space of 18GB.
At the present time Disc C is loaded with Windows XP and a Portuguese Linux Distro " Caixa Magica" ( Dual Boot) this disc usage is 9GB and 5GB free. Disc D ( Data) has used 1MB and 4GB free.
What I would like to do is clear this set of Op Systems - and using WUBI ( I have no CD or USB support to install ) have one fresh install of UBUNTU.
My problem is how to do this - WUBI - says there's not enough disk space. I don't know how to clear down the old systems.