I'm using the same - PT 8 on Windows 7, and I had that issue when I first got it. The manual suggests first turning on Eleven Rack, then connecting it to the computer, and then starting your computer, before a session. And then doing the same thing in reverse when you finish a session.

It's a tad cumbersome, especially with an old computer like mine, that takes a while to start with external hardware hooked up. But it solves the computer freezing problem.