Just happened about 5 minutes ago, after trying to watch a video from the league client my screen went instantly full white and sometimes flickering fast to full black and back to white. When I turned off my computer I got white bars. I am pretty sure it is my video card now since all of this stuff started only recently.
What Catalyst Drivers are you currently using? Have you tried another uninstall / reinstall?
How many monitors have you got?
Are you playing in Fullscreen mode?
Could you try the graphics card in a different pc?
What Power Supply are you using?