Sounds like you have a driver error.
On reboot press F8 until you get into safe mode. Once Win7 has loaded, right click on "Computer" and go to properties. On that window go to the device manager (left side). Under device manager expand your display adapters. Right click on the ATI display adapter and click uninstall.
Reboot your computer in normal windows, and your system will try to install the necessary drivers. You may also wish to go directly to the product manufacturer's download page to install their official drivers.
And don't forget to have a Merry Christmas!