Welcome to SevenForums, i'm a newbie myself. I came over from vistax64.com as i dont use vista anymore.
When i first installed Windows 7 x64 that happened to me once. I was using build 6.1.7100
I restarted and everything ran smoothly from there onwards.
With regards to your problem, I would suggest having Windows 7 as the first option when you're booting up. And also , try booting Win7 into safemode to see if your able to get into Windows at all.