You done just about everything I would have done to fix it! My best guess is that it's a hardware problem given that you've re-installed from scratch. Got anything connected that might be giving grief? Might be worth swapping the keyboard and mouse out to see if that helps. Check the connections inside the tower (although that's not likely to be the problem). Have you installed any new hardware lately?
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Do you know that we're all in line for succession to the throne? Really?
Well, if forty-eight million, two hundred thousand, seven hundred and one people died I'd be Queen.