I have a inspiron 6400 and the wifi will not turn on when i press FN+F2

SOURCE: I can not turn off my Wi-Fi with using fn+f2. when

open your battery and insert it again then switch on your notebook and see

SOURCE: dell inspiron 6400 Vista : the wireless card turns off

is ur WIFI connected thru USB ?
then ur USB is losing power...
check if ur USB setting...
go to Device Manager/USB Serial Bus/USB root hub...
go to each ports one by one....right click on each check properties...check Power Mgt tab....
remove the Tick in the box that says "Allow computer to save power'....
close Device Manager/ Shutdown and restart....

SOURCE: how do i switch on the wifi in my dell inspiron

You can turn on/off Bluetooth and Wireless using the Fnand F2 key.
To enable or disable wireless networking, press <Fn><F2>.
Turns on when a card with Bluetooth® wireless
technology is enabled.
NOTE: The card with Bluetooth wireless technology is an optional feature, so the ordered the card with your computer. For more information,
see the documentation that came with your card.
To turn off only the Bluetooth wireless technology
functionality, right-click the
area and click Disable Bluetooth Radio.
To quickly enable or disable all wireless devices, press
<Fn><F2>.

Please refer the page #69 to 74 for complete information on the Wireless and Blue-tooth

http://support.dell.com/support/edocs/systems/ins6400/en/om/MD402AO3.pdf

SOURCE: The fn key in my

Try resetting bios defaults 1st
You can use an external keyboard and the scroll Lock key in place of the Fn Key, or get a replacement keyboard here:
http://www.ebay.com/sch/i.html?_from=R40&_trksid=p5197.m570.l1313&_nkw=Inspiron+6400+keyboard&_sacat=See-All-Categories