I hooked my new computer up to my NEC Multisync 1700V moniter. When I power the moniter up the GREEN light comes on until I get a "no Signal Message" and then the amber lihgt comes on.

Look for computer warehouse

The only I can think of is the resolution setting that your T400 is putting out that your NEC does not like andcannot display, look at the T42 resolution when the NEC is hooked up to it and try to set it to be the same in the T400.

Look for bad caps with bulging top in the power supply first, post back what you see inside so we can guide you further, see example of failed monitors due to bad caps (after about 2 years): http://s807.photobucket.com/home/budm/allalbums

Does the power light stay on? If so, you have an inverter problem. If the power light goes off, you have a power supply problem.

There's an internal electronic part/s of your monitor that is cold soldered / defective. I suggest to bring the unit to an authorized service center to properly diagnose & fix the problem. regards

check the device manager and see what it says for the monitor...I know you can press CNTRL+ F5 for monitor selection , try this if everything is good in device mngr

the green is working ok but orange is usually waiting for an input from the pc (no input signal)

a blinking green light usually indicates no video signal. check your cables, and if you have another computer, try it on that. otherwise, carefully check your vga signal cable, and the connections on the neck board [where the crt plugs into]

The video card in the computer could be going bad too or the driver for the video has corrupted. Try reloading the video drivers in the PC first.

When you get the red and green leds lit it means the screen doesn't see an input signal and has gone to stand-by. That the red led goes off when you reconnect suggests a signal has been recognised by the screen but be very aware that it may not be a good signal. As you are using a PC input I will assume the signal is RGBHV, maybe you have the H and V swapped around? I would try another computer source into the screen.
You say you are using the digital harness, please explain exactly what this is.