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The Fn key functions as part of the BIOS. I recommend updating this on your computer.
The following are additional suggestions you should take to correct the problem:
Place the System Software CD in your computer. Reset the machine and when the Dell logo appears on the screen, press and release [F2] several times until you are in the BIOS. Press the right arrow key to go to the BOOT menu or hold [Alt] and [P] simultaneously to progress to the second page. Change the boot sequence to CD/DVD drive, Floppy drive, Internal HDD. Now press [Esc] or [F10] to save and exit the BIOS.
The following knowledge base article can help you with this:
How to run the Dell diagnostic test knowledge base article.
The program should automatically load. Choose to run the test on the keyboard.
If none of these help, you may consider contacting Dell Support for a solution.