It would be useful to know what operating system/service pack you're running. Have you tried to boot into safe mode? Its possible a corrupt driver may be causing your problem. Booting into safe mode forces the machine to skip loading all but essential drivers and services.
To boot into safe mode from the power-off state, turn the machine on, then begin incessantly tapping the F8 key. Eventually, you should be presented with the boot menu; use either your number keys or your up-down arrows to select "Safe Mode", hit <ENTER>, and see what happens. It may take a while to boot, you may see a buncha black-and-white text scrolling by before anything else happens, and if it does boot successfully into safe mode, your desktop will look funny - thats all cool. If it gets that far, just shut it down again, and get back to us; we can prolly go from there and get you sorted out relatively simply - relatively, of course, being a pretty relative term when applied to Windows
If the machine will not boot into safe mode, odds are its up for a trip to the repair shop, unless you're comfortable with the idea of opening it up yourself and poking around inside it.
Oh, one thing you might want to try would be to see what happens if, while the machine is not powered on, you disconnect
ALL peripherals, including any network interface, then power it on and see what happens when you try to boot with
ONLY the monitor, mouse, and keyboard connected. Give that a shot too, and let us know the result.