help for balck screen

My computer is old, but the monitor is young. Windows XP, Microsoft 2000. No problems before, but nowdays, I can boot up the computer, the monitor can show the steps just for 1 or 2 sec, but then all black; it can show up the desktop for another second before going to black if I turn off the monitor and re-turn on it. Reboot the computer does not help at all. Can anyboby help what maybe the problem? Thanks a lot!