I would recommend to uninstall first, and reboot as directed. Almost always, a simple "Express Uninstall" of the Catalyst Install Manager (found in Program/Features List of Windows Control Panel) will do. It should uninstall everything 'AMD/ATI', and ask to reboot.
Upon reboot, download and install the current driver you want for the video card. Reboot as directed. If you have a motherboard with an AMD chipset, download and install the chipset driver after that too, and reboot as directed.
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