My dell laptop latitude c540/c640 just stopped working it was working fine then the screen went black i try to reboot and nothing happens no lights the only light is on the power cord

try to change your charger.

there is no slot for a Sim card for the Dell C540/C640 latitude system models.

The keyboard fix is easy, replace the keyboard, same for the battery.
Manual is here
http://support.dell.com/support/edocs/systems/latc610/sm_en/index.htm
you can buy a keyboard here
http://cgi.ebay.com/New-Keyboard-Dell-Latitude-C510-C540-C600-C610-C640-/170342472542?cmd=ViewItem&pt=PCA_Mice_Trackballs&hash=item27a933db5e#ht_2468wt_1202
battery here
http://cgi.ebay.com/66WHR-Battery-Dell-Latitude-C500-510-C540-C600-C610-/280475115213?cmd=ViewItem&pt=Laptop_Batteries&hash=item414d9e9ecd#ht_2340wt_1734
S-Video on that model pretty much requires a direct S-Video to TV connection, then enable the TV in the Display properties. If you are trying to use an adapter, it will not work

several reasons why this will occur:

1) battery is not fully charged - you might try rebooting with the computer plugged into a wall socket.

2) computer's main power supply has been damaged and needs to be replaced.

3) virus - though with most standard virus' , you can still boot up, there are a nasty few that will wipe your entire computer including the operating system.

The drivers for c540 and c640 are different.

C540 Drivers - http://support.dell.com/support/downloads/driverslist.aspx?c=us&l=en&s=gen&ServiceTag=&SystemID=LAT_PNT_CEL_C540&os=WW1&osl=en&catid=&impid=

C640 Drivers - http://support.dell.com/support/downloads/driverslist.aspx?c=us&l=en&s=gen&os=WW1&osl=en&catid=&impid=&SystemID=LAT_PNT_P4N_C640

OK I see C640 and C540 I will give you the link for the C640 go download your drivers burn a CD of of all your drivers for XP for this laptop then you will always have them. here is the link I think you need.
http://support.dell.com/support/downloads/driverslist.aspx?c=us&cs=19&l=en&s=dhs&ServiceTag=&SystemID=LAT_PNT_P4N_C640&os=WW1&osl=en&catid=&impid=

I think you are wasting a lot of effort, I know how you feel and sometimes you cannot win.
Junk it for parts.

I do volunteer work for Technical Aid For the Disabled (a charity organisation) and repair/refurbish laptops and computers for disabled people and I hate to see good computers go to waste.

Windows Vista is not supported on Dell Latitude C540/C640. There are no drivers for the hardware of this model. Many older computers have that problem. You can try Microsoft to see if they even exist: http://www.microsoft.com/uk/windows/products/windowsvista/buyorupgrade/upgradeadvisor.mspx
You might have to fall back to XP.




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Thank You.

ya u can try to chnag ethose how much is the mobo on ebay for one of these also new mobo may be your best bet even fixing them still may not work for u thats 50 50 chance