you can convert your HDD to GPT partition style during windows installation also... follow these steps:
when you arrive at the window where it says to select a partition to install windows on, press shift+F10
now a CMD prompt window will open, now in CMD prompt type "diskpart" without the quotation marks and diskpart utility will get loaded.
in diskpart you must type "list disk", you will see that all of your hard drives are now being listed with a number attached to them. figure out which is the drive you want to install windows on. next type "select disk (x)" x is the number of your hard drive and dont type with the brackets. you will get a message that disk x is now the selected disk. now type "clean". once you get a confirmation that its cleaned type "convert GPT". thats it! now your hard disk has been converted to the GPT partition style and now close the CMD window and proceed to installing windows. you may have to refresh the window where your hard drive partitions are being listed. hope this helps!