keyboard problems with xp service pack 2

i just installed windows xp service pack 2 on my dell latitude c640. i
am now experiencing keyboard problems. sometimes certain keys all of a
sudden stop working. for instance, today my 'e' key stopped working.
last week i could not use the enter and spacebar keys. shutting the
system down and rebooting always solves the problem, at least
temporarily. the problem only occurs with the built in keyboard, i
never experience the problem on an external keyboard that is connected
to the port replicator. i never had this problem prior to installing
service pack 2, so i am pretty sure this is the culprit. any one else
seen this problem? any ideas on how to resolve it? thanx.