I know it it may sound foolish but laptop keyboards can be sensitive to any stray and accidental contact to the touchpad. A user may not even be aware that they've touched it. One way to tell is to disable the touchpad and work strictly from the keyboard for a while. If the problem goes away, you've found the culprit.
I have a Dell Latitude 5400 laptop. I use Internet Explorer 8 with Windows XP Professional. Microsoft Security Essentials scanned and reported no problems with my hardware and software.
But, the cursor jumps around when typing, sometimes letters do not appear when typing, and sometimes content is erased when typing.