My Local Disk D in Windows Vista has a Volume of 8.30 GB.
The Used Space is 8.01 GB, leaving 291 MB of free space.
This is not even enough for 1 back-up of the System.
What can I do, if anything , to free-up space?

What are you using this D drive for and can you simply delete anything that's there ?

I suspect that D drive is your Recovery partition and should not be used. External USB drives are cheap.

Frankly, I do not know how it got so full. Unless I added or down loaded something to it by mistake.
Can I go in there and delete the stuff?

Did you read what I said? On most new machines there is a small partition, usually about 8 G, that contains an image of your system as it came from the factory. This is your Recovery partition. And you should leave it alone.

Thanks, ScottGem. Gottcha!