Screen Goes Dark, Computer Still On

I have a Dell Inspiron 1525, running Vista home premium 32 bit. I've had this laptop for about 3 years, and for the past month or two, the screen will at times go blank (for instance, if the laptop has been unplugged), yet the laptop itself remains on. The problem is sometimes I can't get the screen to come back on, even though the laptop is still on and running. The only thing I seem to be able to do to get the screen back is to turn off the laptop by pushing and holding down the power button, forcing a shutdown. When I reboot, I get a message saying that Windows did not shut down properly, and have the option of entering safe mode or starting Windows normally. I select start Windows normally, and everything seems to be more or less OK, until sooner or later this same screen issue happens again of course. Any idea what's going on here and how to correct it? I've recently done a bunch of maintenance on the computer (cleaning the registry, clearing up some hard drive space by deleting a bunch of larger files, etc. Any help would be much appreciated. Thanks.

Lance