Let's start from scratch by deleting everything first. If you have a world save then copy it to a another location then follow the steps below.
1. Backup your world save if you have one
2. Delete the minecraft folder from the desktop
3. Delete the ftb launcher folder from roaming
4. Download java from the link
here and save it to the desktop. Now download javara from
here and run it to remove java then reboot. Now install the one you downloaded and reboot again.
5. Create a folder on the desktop and name it FTB
6. Download the ftb launcher exe from
here and save it to the FTB folder you created on the desktop
7. Run the launcher and make sure that the install path is set to the folder you created
8. Download the modpack
Let me know how you get on.