Hi! This should not be problematic. If you select the option to install GRUB to the mbr of your boot disk (during the installation of Ubuntu), Ubuntu should detect all the Win installations and add them to the GRUB start menu. Ubuntu must be the last OS to be installed for this to work. Your boot.ini files will not be altered. Similarly, the operation of NTLDR will remain the same after you have selected a Windows OS from the GRUB menu.
The only time that I have encountered difficulties is with Ubuntu on a second HDD - You have to be careful to install GRUB in the mbr of the disk that is specified in the BIOS as the boot device.
It's also relatively simple to install Ubuntu (with GRUB installed onto the Ubuntu partition, not the boot disk) and then add Ubuntu to the existing NTLDR setup. There are various guides that Google will find. It's less difficult than it looks, but assumes that you are happy to use the Linux Terminal.
Edited by byteback, 21 March 2011 - 05:20 PM.