Black screen :(

Hello,

When I start my computer it passes post and the windows screen appears however after a few seconds the screen goes black/blank. I have switched monitors and I get the same problem. It will not start in safe mode, or any other mode. The A/B/C/D lights on the back of the CPU are all green. I have tried to boot to the last known good settings and still get the black screen. This is my mothers computer and she knows very little about them, she has changed settings and god knows what else tring to fix a internet problem, she spoke with dell who told her that the hard drive was bad however she says that the computer was working after that point, she spoke with the internet provider who told another part was broken...... I am pretty good with computers and can change out parts however I was hoping someone could give me some direction before I resort to buying and replacing pieces without knowing if that is truely the problem. I have read others posts about a black screen problem however the only advice I seen was to turn the computer on and off until it worked?????? that did not work for me. I have tried to reinstall / fix windows however the computer does not register any CD I put in the CD ROM. This is a Dell Dimension.

Thanks