Do you need more than one partition to run an Ubuntu installer? Or can you download it, save it, and install it all in the same partition? (without mounting others, nor using the Windows installer).
On the partition in question there is a pretty old Ubuntu installation, but it is enough to get the installer started. The graphical stuff is not running in this version, but the command line is OK. It doesn't matter if this version gets completely deleted and there is something like that 40 GB free.