Your experience is common with ATI drivers. The original driver had nothing to do with your laptop being slow. The built-in video uses an ATI chipset but it is not necessarily identical to a retail product. The computer manufacturer (Sony in your case), provided an OEM driver designed for what is inside your computer. ATI periodically offers updated drivers, but these frequently do not work with OEM hardware. I'm only guessing that remnants of the new driver remained when you tried to reinstall the old one. When you disabled it through the device manager, the computer was running with some basic video drivers, which is why it looked bad.
The only place to get drivers for the OEM hardware is from the computer manufacturer (unless you can't). Here is a Link to Sony's download page. You will need to find your particular laptop, which you didn't identify in the question. Download the latest driver (don't be surprised if it hasn't changed from the original; there is rarely a need to change the original drivers).
Re-enable the display adapter in the device manager. Completely remove the ATI drivers following the instructions here from the AMD web site. Although it isn't mentioned in those instructions, you may need to do the uninstall in safe mode. Then install the driver downloaded from Sony.
If the problem remains, it is likely that all of the remnants of the new driver were not removed. There are a number of third party removers. AMD used to have a separate removal program. Some online discussion indicates that it may still be available, buried on the ATI driver downloads page. From the comments, I understand that you tried the one covered here successfully, running it in safe mode.