From the size it sounds like you did a Wubi install inside the Windows partition, not a a full install in another partition. Have a look at this question which asks how to resize a Wubi install. : increase-size-of-root-partition-after-installing-ubuntu-in-windows
It might be better to install properly on that spare partition rather than continue to use Wubi when you don't need to. You'll need to boot off the Live CD or USB to install Ubuntu but before you do boot the Ubuntu that you have installed and check which disk the root file system is on. As per the question linked above, open a terminal window after Ubuntu boots and type the following:
and paste the results here so we can see.
From the result of you 'df -h' you can see that Ubuntu is running off an 18GB Virtual disk that is in the Windows partition. In other words, you did a Wubi install, you did not set up a dual boot system with Ubuntu in its own partition.
I suggest you boot off the LiveCD or LiveUSB and do a full install of Ubuntu alongside Windows.
I highly recommend that you back up all your important data before you do this. While it's unlikely anything bad will happen if you are very careful, it's good piece of mind to know that your data is safe if something bad does happen. Chris