Sound like you need to install the graphic drivers. I'm a little surprised that the Vista drivers didn't carry over, if you did an in-place upgrade to Windows 7. (Or did you do a clean install?)
Because notebook makers tend to use custom device IDs for their graphics adapters, the place to begin is with the manufactuer.
HP doesn't list Windows 7 drivers for that notebook, but you may be able to use the Vista drivers: NVIDIA GeForce Series Video Driver HP Pavilion dv6735en Entertainment Notebook PC - HP Customer Care (United States - English)
If that succeeds, you can probably get newer drivers through Windows Update, or directly from nVidia: Drivers - GeForce
You can also try the nVidia drivers before trying the Vista ones from HP.
Good luck. Let us know what works.