I'll have to apologize for a few of the posts on this thread it appears that some of the users didn't understand your situation. If you're getting no video as you turn on the computer as you mentioned you're not going to be able to get into Safe Mode and you're not going to be able to download drivers for the hard disk drive.

You'll have to first diagnose the video or system startup related issue.

It is possible that your hard drive could be causing this issue. However, generally with hard disk drive ISSUES you'll encounter some type of error message and not no video. Before saying exactly what hardware is causing your system problems lets first try to determine what hardware if any is bad.

When your computer starts you say it seems as if it's going a sleep. First, try disconnecting everything from the back of the computer (keyboard, mouse, network, etc) except the monitor and computer main power and try turning on the computer to see if you get any display.

If you do not get a display try disconnecting the main power, waiting a few seconds and then reconnecting the main power. Try turning on the computer again.

If still no display, try another monitor on the computer, it's possible it could be a simple as a bad monitor.

Finally, if still encounter issues could you let us know if the computer is doing any type of beeping when it starts up?

I have tried all of those things including a different monitor which I took off of my working computer and have had the same results everytime. No display on the monitor, the green light COMES on for a second or two then GOES to yellow. I do not hear any beeps when the computer is starting and it seems all the fans and drives run. I think I hear the hard drive spinning, I hear fans come on when the computer has been on awhile, and when i put a CD in the CD slots they start spinning. But never any display on the monitor, and once the light turns yellow, it never goes back to green.

Thanks again for all the help and advice and any help in the future.Suggest you give "Mitz" another look. Her ARTICLE on this subject is worth a look and very well written.

http://www.tips4pc.com/Articles/Computer%20Troubleshooting/computer_has_a_black_screen_and.htm

Careful: If you reseat the CPU and heat sink/FAN you might need to reapply a thermal paste between the CPU/heat sink to prevent the CPU from overheating.How familiar are you with working on the inside of your
computer case?

Do you know how to assemble a computer?

If so,Is the power supply fan and all other fans working?