I have a Dell Inspiron 8600 laptop (fairly old), with fully updated XP. Today when I tried to start it up it presented me with a black screen instead of the standard blue welcome screen of XP.
After some restarting I found that it would allow me to login whilst on battery power, but if I tried to plug in AC power while on the desktop the screen would go extremely dark (so that you can just make out the icons). Turning up the brightness in the usual manner has no effect.
If I try starting it up with it plugged in, Safe mode/VGA/last known good settings etc. in the f8 boot menu all lead to the same black screen problem.
Another thing I noticed, if I'm on the desktop (whilst dark) and either alt-tab to a full screen game such as Age of Mythology, or change some display setting like the resolution, it will go back to normal for about 5-7 seconds before going dark again. A similar thing occurs if I let it run out of battery, then plug it in and start it, it will show the welcome screen, I login, and soon after the desktop appears it goes dark.
I've also tried putting all the power saving options to 'never'.