As no one has volunteered any answers, I'll make a newbie stab. Have you checked out?
I assume this is an XP desktop. When the pc starts, the screen is ALWAYS black? Like you don't even see a 'bios' screen for a second or two? Have you tried starting in Safe Mode? Have you opened the cabinet and "dusted" everything and pushed/reseated cards? I turned on an older pc a few days ago and the 8 year old 15" monitor was blank. I tried the monitor on another pc and still blank. I put the monitor in a corner for 24 hours. I'm using it now. Go figure.
8 years seems a long, useful and happy life. It may just be time to say good bye.