It should be there already... try the following:
Originally Posted by rsearjeant
Boot your computer, select Ubuntu from the Windows boot manager, when you see the Wubi grub menu hit 'c'. This drops you to a grub prompt.
Now load up your migrated Ubuntu's grub menu (assuming /dev/sda5):
This is exactly what you will see when you install the grub bootloader. You can then test out the Windows option and the rest before installing grub.
If you're still nervous then backup your MBR before installing the grub bootloader. But reinstalling the windows bootloader is very simple - I just use lilo even though I have the windows disks because I can install it from Ubuntu, and I've never had an issue using it.