what could be wrong with this dell?

i have a friend whose dell 4100 just gave up on him after he put new ram in.. and then he put old ram back in and it did not work.. so today i went over put a stick of ram from my computer in there and it didnt work.. so then i tried putting a different video card that works on my computer just fine and still does not work.. but when you power the computer up theres no video output but the keyboard works fine like the num lock and caps lock buttons...
is the motherboard fried somehow? i mean it couldnt be the powersupply.. those are the only two things left.. we also tried unplugging components to check if the power supply was overloaded

max

Did you try a different monitor?

Dell computers are very picky about RAM. But if he has problems with the original RAM, it could be the motherboard. What are the diagnostic lights on the back of the 4100 showing. I had a motherboard go on a 4100 and it showed a RAM error for about 2 minutes. Then the lights would switch to a video card error. You can find the error codes on the Dell website. You have to sign in first.
http://docs.us.dell.com/docs/systems/dzuul/codemess.htm

hmm. right the lights in the back.. i didnt check those.. but the monitor works fine with his laptop i tested

max