Cannot find a device driver for dell latitude c840

I went through quite a maze to find this. Maybe I took a wrong turn somewhere because I don't know how you ended up with this device in your laptop. It won't hurt to try it though.

http://support.euro.dell.com/support/downloads/download.aspx?c=uk&l=en&s=gen&releaseid=R151517&SystemID=LAT_PNT_PM_D610&servicetag=&os=WW1&osl=en&deviceid=6483&devlib=0&typecnt=0&vercnt=14&catid=-1&impid=-1&formatcnt=1&libid=5&typeid=-1&dateid=-1&formatid=-1&fileid=202136

Please note that True Mobile 1400 is the wireless module, not the NIC-interface.
The driver for the 1400 here: ftp://ftp.us.dell.com/network/R143355.EXE

If you're not sure if the loaded driver is correct, you should 'uninstall' it in the device manager.

you need to know the exact hardware to be able to install the correct driver.
To find more info about the exact hardxare there are a number of free good tools:

i usually use : Aida, Siv (system information viewer) or siw (www.gtopala.com).

if you know more info about the hardware it's best to try the dell drivers first,
not necesseraly the exact model, eg i found your truemobile 1400 driver with the latitude d600 (the next model after the c-840).  only if you feel adventurous you should try the original vendor, often those drivers are for more recent hardware and are unnecesary (in worst case less stable) for your hardware.

hope these tips help.

Presuming that the laptop is in "as shipped" condition, meaning that you haven't changed out any of the original hardware, try this.

Go to Dell Support at:

http://support.dell.com/support/downloads/index.aspx?c=us&cs=04&l=en&s=bsd&~ck=GuidedPathHomePage

Select Choose by Service Tag and enter the Service Tag number for your unit. The drivers that show up should be the ones that will work for your laptop.

I really appreciate the help I have received.  However, my customer wanted her laptop (I plugged in a wireless card for her and let her use that temporally) So I will have to wait until she brings it back so I can try what has been suggested.  I also failed to mention that the service tag for this laptop is 5JLRX21 and all the drivers in the network section did not work.  I did not try any of the utilities, Html or firmware, only the drivers.  I tried all of them.  Anyway, I am anxious to try out what you all have suggested when my customer brings back her laptop.  I will let you know how it turns out.  It will probably be tomorrow or the next day.  

After you find the correct Drivers and Downloads page using the directions provided by souseran, refer to this article - https://www.experts-exchange.com/viewArticle.jsp?aid=329 - for the correct load order. Note you may have to follow the 'boot in Safe mode' directions if you have not already loaded the system software and chipset drivers.

Changing the setting in bios did in fact fix the problem.