Yes, you shrink the C: Partition. You have about 400 GB of free space available, so you should be able to shrink it by about that much. Once you have done that, create a new partition, move everything you want to save to your new partition, and install Windows 64-bit on the original C: Partition. Afterwards, simply take all the saved data, still on your new partition, and transfer it over to your new Windows installation.