I recently replaced the motherboard in a Dell Latitude D620.
There is a known issue with the Nvidia video chip on the D620 and D630 model and Dell seems to be willing to replace the motherboard if you are less than one year out of warrenty.
Here is my step by step that I used in conjuction with some videos on the proceedure I found at Google video.
Dell D630 D620 Motherboard replacement
Take out Battery, Hard Drive, Memory
1. Remove all the screws on the back
(Under hard drive are two more screws)
(keep track of big vs little screws ... there are four little screws back two corners and LCD anchor holes marked D )
2. Flip Laptop
Take out keyboard
2nd memory out
WLAN cabling out
LCD cable unplug
3. Take out P labeled screws - 3
4. Take our palm rest - I start with the section by the hard drive bay which is loose, then work the back end of the palm rest by the ports ... do the front last as you can slide the palm rest forward to loosen the front edge. IMPORTANT: Read these three items below:
* Wifi hardware switch mush be in OFF postion
* Also battery connector wire near WLAN radio MUST be in unplugged - little black and red wire
* AND Disconnect the touch-pad cable connector from the system board as you pull the Palm rest up
5. Take out speaker
6. Heat sink out - four screws in order 4,3,2,1
7. CPU out - twist screw to loosen (note position and put immediately into new mobo)
8. Remove modem if installed
9. CABLES: detach hard drive ribbon cable / detach cable next to WLAN and serial port / detach fan power cable / detach very small cable next to modem and
10. Motherboard screws: 4 with little white arrows next to them
11. Unscrew anchor screws for video port and serial port - use 3/16 socket
12. USB / Modem / NIC connect to motherboard via cables - unhook and or remove
13. Take out motherboard
14. Insert new motherboard ... keep an eye on little cables
(A plastic cover over the USB ports had to be removed on my replacement motherboard)
Then just go backwards through these instructions to reassemble the laptop.
NOTE: When replacing the Heat Sink I cleaned off the old heat sink putty with the cleaning wipe provided and put on a new strip of provided heat sink putty. VERY IMPORTANT: Screw back in the heat sink screws in numerical order! 1,2,3,4! Firm but not too tight.