i have a dell desktop.. i restarted my computer the other day and when it would not restart..only stay on the logging off logo..i maually shut it off. now when i try to start it.. it makes a real loud humming noise like a fan..and all i get is a yellow light on the power button and it does not start.. i checked the fan that is in the front.. over the cpu i think.. it is turning but was covered with dust on the solid screen in front of it.. any suggestions? it is a dell xps 420. the sound sounds like it is coming from the fwd fan or near it.. thanks!!!