I've been fighting my own Dell problems the last few days. I was given a friend's old computer, which is newer than what I use. I bought a HDD for it, plug it in and try to test it. I get no picture, and some flashing power light code, which I paid no attention to. After a few attempts with turning it off and on and messing with the screen, it finally shows something: a PXE error. I fix that in BIOS, but get a no boot device message. I fixed that in BIOS, and it all works, and at some point I even ran the self diagnostic feature and set the processor to always run at full speed.

Then, I fully install the HDD in it's slot, put it all back together, and plug the computer in, and it starts and stops repeatedly. I try unplugging it a few times before researching some solutions. Next, I replace the coin battery, and then it stayed on, but no image, and a flashing power light code, which is impossible to tell which of 2 possibilities it is. It's either 2,3 (a memory, cpu, or motherboard problem) or 3,2 (a video card problem). Well, I have no video, but I pull the RAM sticks out and put them back in. Now, it starts up normally, except no video still, and it shuts down after 5-15 seconds. Then I took the heat sink off to find the thermal paste is dry, which shouldn't cause that bad of heat transfer in my opinion if the heatsink was left in place. I'd think the fan would go to full speed if ever the processor got hot. I decide to start it up with my finger on the bare processor, and sure enough it got too hot to touch in 10-15 seconds. So, tomorrow clean it and apply new paste and see what happens. I'm still not sure why the video doesn't work.