If you are booting and you see the GRUB loader offer you a choice, then at some point, you chose to install ubuntu to the disk. The ubuntu install most likely took a portion of the disk for itself leaving the remainder for windows.
So you have 2 tasks here. #1 Remove the Grub loader and use windows to boot. #2 Remove the ubuntu partition and reassign to windows.
**** Make sure you backup anything important ****
Anytime you go messing with your disk like this, make 100% sure you have copies of whatever data you can't live without.
HAve a look at this site for easy instructions on removing GRUB and reassigning the space: http://www.wikihow.com/Uninstall-the-Grub-Bootloader-from-a-Dual-Boot-XP-System-With-an-XP-CD
I've used paragon partition manager for stuff like this with success many times. ... FYI