It wasn't like this before... and no matter what I do (changing the colour settings in display properties or using those little menu buttons on the front of the monitor) the greenness is still there! Aaah
You haven't got anything else electrical in the very near vicinity have you? Something that could emit a magnetic field? I don't know what it does to a TFT but the TV in our kitchen has a green tint from being sat next to the kettle for about 3 or 4 years.
Ive seen this a lot in the heavily abused PC's at the school I used to work in. More often than not it was fixed by either making sure the monitor cable was connected ok, replacing the monitor cable, or reseating the graphics card.
If youve got access to a different computer somewhere, try the monitor on that one to see if the problem persists. Then at least you can see if its the monitor, the cable or the graphics card thats the problem.
I made two CRT's pretty useless by putting them near something magnetic fieldy.. I never worked out what the problem was.. But it hasn't affected my LCD (Yet)... A fun thing to do if the monitor is fucked: Monitor + Sledgehammer = Destruction..
I reseated the card like mr Joose suggested, and the monitor decided it didn't want to recieve any signals. I turned it off and plugged it into the second connector on the card, it recieves signals now but is still greenish.