Keyboard jumps backwards while typing

My laptop computer gives me fits when I try to type. As I am typing, the cursor jumps backwards or to another line. I experimented with gmail and Word 2003, and both of them produced the same result. I plugged in a USB keyboard and the problem did not recur. Question – does the laptop have a bad keyboard, or does this problem come from the OS. I can only fix the OS. Dave Weiland