I just installed a MAG 17 incuh flatscreen monitor to my Dell Dimension 3000. When I turn the PC on, the monitor disploays everything on the desktop,but then goes black. If I turn the monitor on and off, it again displays the desktop, but goes black again. Thoughts?

I think that you have chosen some settings that are not compatible on it ..resolution too big and probably the refresh rate high ..
first from the monitor settings ..press the auto-settings button ..and let it make the proper choice ..after that ..to eliminate any settings in the video card that might affect it ..boot your computer in safe mode..and remove the video card driver from control panel ...add /remove programs ...and after that run it again from the disk or ..from teh support site if you do not have the disk ..
If you only have the on board driver..and you do not have a disk ..run it from the drivers of the motherboard disk ..This will reset the odd choices on the video card ..and make it work ..to open in safe mode..press F12 on start ..and choose safe mode..(for some computers the button is F9 or F8 )Usually the button is specified when it boots up ..
But ..for to be sure that this is the cause...you have to hear to computer running ..and only the display to be black ...otherwise can be a physical fault..