To remove Ubuntu from the Windows Dual boot setup you will initially require a Windows installation Disk in order to reinstall the Windows Master Boot Record after removing the Ubuntu system.
If you don't have a Windows Installation Disk to hand you can create a Windows repair disk from within your active Windows system (presume it is Windows 7) via the following instructions:
http://windows.microsoft.com/en-gb/windows7/create-a-system-repair-disc
Note, after creating the Repair disk perform a disk Bootup test to make sure it works ok, before attempting to remove Ubuntu. A 'repair options' menu should display similar as per the following (ensure it has the option 'Command Prompt' included):-
http://windows.microsoft.com/en-gb/windows/what-are-system-recovery-options#what-are-system-recovery-options=windows-7
Next stage is then to perform the steps to remove Ubuntu as per the following guide:-
http://www.makeuseof.com/tag/how-to-safely-uninstall-ubuntu-in-windows-dual-boot-environment/