Windows bootloader shouldn't use all of the 100MB space.
Probably the installer detected that your boot efi partition is for example 99,8MB large so it's not enough (the installer minimum limit is round 100).
What can you do?
1. Run Sparky live iso
2. Update package list:
sudo apt-get update3. Remove the present version of the installer:
sudo apt-get purge live-installer4. Install the latest version of the installer (20150204) from sparky repository:
sudo apt-get install live-installer5. Edit the file via a text editor (with sudo), for example:
sudo apt-get install -f
sudo leafpad /usr/lib/live-installer/frontend/gtk_interface.pyChange 'leafpad' for a text editor you have installed.
Find line 829 and change '100' to '80', save and close the file, than run the Sparky Installer.
6. Let me know does it help.