I believe that particular driver had it's own installer. If you look in your program menu (Start -> Programs) there should be an ATI subgroup there should be an Uninstall shortcut from there. If it's not there you can look for it in C:\Program Files\?? assuming you installed it into the default path supplied when you opened the download and went through the installation wizard.
It's strange this is worse, because that's the driver for the card your computer thinks it has. It's difficult for me to help without knowing precisely what graphics card you do actually have. Do you have any of the packaging? and it would have come with a driver CD Rom if you have mislaid those you will have to open up the PC and get a good look at the card for a model number so we then know exactly what we are looking for.