I jave disassembled my Casio CDP 100, to check a couple of dead keys, how do I remove the key wothout breaking it..

SOURCE: Need replacement key set for Casio LK-110

You would need to call casio for such parts or go to a part store which by googling it you can find the parts for this unit.

SOURCE: I have a CDP-100 and need disassembly instructions

I just fixed my CDP 100 which had a similar problem - dropped and two keys not working up one end.
1. Lay keyboard upside down on soft surface. Under keyboard is plastic cover held on by 17 screws. Remove all these and lift cover away. The metal key ends are all revealed.
2. Locate the disloged keys. Carefully press them back into position - they will "click" into place. Make sure the corresponding key on the keyboard isn't stuck in an "up" position - you may have to remove the metal key part and replace it again while making sure the key is correctly seated.
3. Replace cover and screws.

SOURCE: I have a Casio cdp 100 keyboard and two of the keys are stuck in the down position. Almost like the spring has gone in these 2 keys. Can this be fixed and if so, how?

Ordinarily there is a separate spring on each key attached to the rear of the key that sometimes pops loose or the attachment point breaks off. However as Casio describes this as "springless action" that leads me to beleives it has a weight attached to the key instead of the spring setup. ROland used to use that sort of action and had a lot of problems with the weights coming unglued.
At any rate You would need to take the instrument apart enoug to get to both sides of the action top and bottom. Frankly, it's no fun and I'd suggest taking it to. An authorized Casio service center. You may find it's a recall problem, but if not, the service center will have a better chance of getting Casio to ship them the parts than you would.