5.1.5621 is the right driver set for these Macs. It's never booted into Windows, when that is selected in the Startup Disk preference (in either OS), regardless of Windows version etc. PRAM reset has no effect.Is that an older driver set?
Odd that it's hanging... and, I don't think it would have anything to do with the fact that it's a flashed 4,1. My own machine is a flashed 4,1. The one time where I couldn't boot back into Windows was after the latest Bootcamp drivers, for some reason, failed to install (without my knowing). The drive showed up, but it wasn't bootable.