This tweak is very dangerous and requires reading carefully. If you do it wrong, your system will no boot or you can loose everything. So make sure you backup your computer and read carefully.
To correct the dual boot problem do the following:
Go to my Control Panel - > System - > click on the Advance tab go to Start up and recovery section and click on settings. Where you see the default operating system choose one and click on edit.
Here is where you have to be careful you need to delete the option of operating system which you do not want to use any more or is invalid. Click OK and exit.
If you did everything correctly and read the instructions on the links then your system should boot up now to one operating system.