I believe the GA-P55-UD3R motherboard has two separate controllers. I suggest you attach your boot drive to one controller (GSATA2_0/1) while it is set to IDE and configure the controller that is not being used (SATA2_0/1/2/3/4/5) to AHCI. You should be able to boot just fine with your drive plugged into the controller set to IDE. After you successfully boot to windows, install the AHCI drivers for the controller that is set to AHCI. Shutdown. Now, attach your boot drive to the controller that you set to AHCI and boot.
I've done the controller switcheroo on my Gigabyte board to switch from IDE to AHCI with success. Hopefully it works for you to.