It did it again! I copy paragraphs or lines of text. I was used to holding Shift and using the arrow keys. It seems to have something to do with the Shift key. I don't type properly and sometimes I touch more than one key. It seems to be to the left of my keyboard (French Canada) with A, S, D...
I have the same problem Windows 7 and Comfort 4500 mouse (wired) on a Dell Vostro 1710. I restored the defaults on the mouse and will disable touch pad, which I HATE (useful on the road, though), See I had "Comfort Mouse"in my Clipboard and my PC stuck it my itself.