I'm having the same issue. Got a new computer and want to install my Windows 7 Ultimate 64bits on it, but the HDD came formatted with MBR. Being a 3tb HDD, it has 1tb unusable, so I want to convert to GTP.
I did what youre telling, but since the disk is already formatted, Windows install (from a boot from the DVD) simple dont offer to format, there isnt this option. It refuses to let me delete the working partition, the delete option is gray. Its infuriating. It has the 2tb partition and another 765gb unused and it doesnt let me do anything with this second one.
I started Dariks Boot and Nuke, it seems great, but it will take only 40 hours to finish!!!!
I wonder if there is a tools disk to do this formatting like DOS had the FDISK (I had a DOS diskette with everything necessary to format/install an HDD, was so easy), I simply cant believe this is such a difficult task to perform now!