keyboard/mouse problem on laptop

[fastnsmooth]

Recently the keyboard and touchpad on my Presario 2140US (Athlon 1.4 Ghz, 512 MB RAM, XP Home Edition SP1) began producing random repeats of the letter m. Initially, it happened rarely, and I just deleted the extra letters. It has progressed to the point that, as the computer boots to the Windows page requesting my password, an endless stream of m's prints spontaneously. Sometimes the delete key will stop the keying and remove the characters, sometimes it will not work. Additionally, if I make it to the desktop, icons begin spontaneously highlighting in very rapid sequence, without any input from me. Sometimes this erratic behavior is soundless, but sometimes it is accompanied by a rapid "bonging". Clicking the touchpad left key sometimes stops it, and sometimes has no effect. I am very certain that nothing has ever spilled on the keyboard. I bought a PS2/USB adapter and plugged in a regular keyboard and mouse. Everything was fine for about 2 hours, then all of the same problems began again. The laptop is out of warranty. HP's online tech support said replace the keyboard, but I am concerned that that won't solve the problem, after the way it reappeared with the external keyboard and mouse. This unit has never been connected online, so I've ruled out a virus. I searched old threads here, found a few questions about repeating keys, but those seemed to be solved with new keyboards or cleaning stuck keys. One question occurs to me: is it possible for a short or a defective key in the integral keyboard to override the external keyboard/mouse?

I apologize for the length of this post, but I wanted to be sure to explain the problem completely. And yes, I've tried rebooting: no effect. Any assistance will be appreciated greatly.