If the Ubuntu installation has mostly completed, I recommend you try one or both of the following:
- Download a USB flash drive or CD-R image of my rEFInd boot manager, prepare a disk, and boot from it. You should see options for both Windows and Linux. If both work, then you can install the Debian-package version in Linux and forget about GRUB.
- Run the Boot Repair tool from an Ubuntu live CD. This should install a fresh copy of GRUB, finishing what the Ubuntu installer failed to do.
If both of these fail, then post back with the URL that Boot Repair provides, along with a description of any errors you see from both approaches. That will give us more information about what might be going wrong, enabling a better diagnosis.