Dell Inspiron 9100 Fn button half of the key not working

SOURCE: Dark Screen Inspiron 9100

If you replaced the inverter board and its still dark you need and new backlight itself http://cgi.ebay.com/LCD-Backlight-Lamp-for-Dell-Inspiron-9100-15-4-WXGA_W0QQitemZ150217509448QQcmdZViewItem

SOURCE: Dell inspiron xps 9100 cpu upgrade

Highest compatible cpu for your system is the Pentium 4 3.6 ghz Prescott. Hope this helps & please rate this solution fixya. Thanks.
Here is a link to a spare parts supplier for your system: http://www.dellweb.co.uk/spares/inspiron/9100.htm

SOURCE: fn key on dell inspiron 1501 keyboard not working

Sounds like your number lock is on. Find the key with blue lettering that says 'Num Lk' (or something very similar). Press Fn and that key, and your number lock will be turned off and your keys will go back to normal.

Hope this helps.

SOURCE: laptop key repeats or doesn't work at all

what you can try to do is to test an external monitor. If it works fine, try to check for any stuck key on the keyboard and any foreign objects. If all is well, you can request a replacement keyboard from dell if your system is still within warranty.

SOURCE: how to turn off the number lock button

Just press Fn+F11. An icon of a Padlock with a number 9 must be unlit for you to use the keyboard normally. If you need to activate the numeric keypad again, just press Fn+F11 again.