You have done the WUBI installation. Wubi has some limitation. Wubi installs Ubuntu on a large virtual partition that is a single big file called the root.disk. In your case, this file seems to be 20GB and is located in the 100GB partition you created.
What to do?
Ubuntu has its own partition format ext4
and can't use the format used by Windows. To install Ubuntu in its own partition, you have to create a live DVD/USB and boot directly from the DVD/USB.
Before you boot from Ubuntu DVD/USB, you should delete the 100GB partition while you are in Windows. This will create 100GB free space that Ubuntu will recognize and create its own set of partitions in.
See the following links at Ubuntu.com for detailed instructions:
How to make Live DVD.
How to make Live USB.
Steps to install Ubuntu.
Also see:
Ubuntu installation on Windows7 with D partition
I have formatted a partition for Ubuntu. How do I install it on that partition?
Dual Boot Installation with Win7 - Install Ubuntu in New Partition
How do I install Ubuntu?
Hope this helps.