Unity3d 5.3.1f1 Grey Screen trying to open editor Windows 7

I'm trying to install Unity 5.3. I previously had Unity 4.6.1 installed which had opened and operated correctly. I've tried installing 5.3 with both 64 and 32 bit versions. Each installation has completed "successfully" until Unity is launched. When Unity\Editor\Unity.exe is executed a splash screen appears for a couple seconds followed by this grey screen:

The process isn't using any ports, it reads from and writes to disk @ ~2k bytes/second for 3 or 4 minutes and then stops. It doesn't display any progress bar or error message and I cannot find a log with an error in it. I don't really know where to go from here. Anybody know what's going on?