I just re-installed Win7-64 and all my software, including Master collection CS5. I started a new project in PPro CS5 and noticed that video rendering is set for "Mercury Playback Engine Software Only" and is greyed out, so I cannot change it. I have a GTX 470 and it was working just fine before I re-installed windows. I did add the drivers for the vid card just like I did before. Is there something I set incorrectly when I re-installed CS5?
Make sure you have the very latest Nvidia driver. The GTX470 was added to the support list a while back. Maybe you were using an older driver with the hack before you did the re-install?
Also check to make sure that the driver install was good by checking the driver version in the Nvidia control panel. If its using the default windows driver it will not work. I had this problem and had to re-install the Nvidia driver for it to work
Did you upgrade CS5 to the latest version?