One of your keys on your keyboard is stuck down
When it starts shaking again, don't touch the keyboard, just open Notepad (or wordpad) from clicking on the Start menu->All Programs->Accessories->Notepad
You should see the key (ie the letter usually) repeat itself across the document, until you stop it by pressing any key on the keyboard
Try cleaning out the keyboard (turn upside down and tap underneath) or even vacuum it ! (or possibly replacing it)