Macbook shuts down when unplugged from AC adaptor

Until yesterday my lovely macbook was motoring along smoothly. Currently it is showing a 99% battery charge and each time I unplug it from the AC adaptor the screen dims for a few seconds like it is going to sleep and then it shuts down. The only way to bring it back from this is to plug it back in and turn on the computer via the power button. I purchased this computer in 12/06 so it is almost 2 years old but have never had battery issues.

I read many other posts on this topic but all seem to be unanswered. I tried the SMC reset, tried to reboot by pressing the power button for 10 seconds, and tried to let the battery run down by putting it to sleep and running off of the battery. All of which did not work. Any other suggestions would be much appreciated.