keyboard gone crazy... something affecting fn key???

my keyboard appears to have gone crazy. it appears that my function key is permanently activated, although the computer passes the diagnostic stuck key test, and the key is visually okay (even after i popped the top off to make sure everything was looking right). basically, the only keys that the computer is picking up any values for are the keys that i can use as the number pad, plus a few other keys with function values. the keys that i can use as a number pad give me the values that one one normally find with num lk turned on (the arrows, home, end, and page down most of the time. sometimes i can also get the numbers... but never the letters, and i'm not sure what i do that makes the numbers all of a sudden work).

also, since this happened, when my computer starts up, it automatically boots in diagnostic mode. this does not happen when i restart. additionally, i can successfully use a usb keyboard, no problem. these two signs had me believing that this was probably a hardware problem, not a software problem, but just to be sure, i reinstalled my operating system. sure enough, this didn't fix the problem. anybody have any clues as to what is causing this, how i can go about fixing it, and what it's going to cost me?

thanks!