Restore points are a great thing to do.
Probably the easiest and safest way to try to do something about this is to use Windows' built in disk cleanup program. If you click the start button, then all programs, then accessories, then system tools (they could definitely have made it easier to find), there will be disk cleanup. Start that and tell it which drive to analyze (D in your case). It'll look for things such as files in the recycle bin (Windows never really deletes anything till you empty the recycle bin) that can be emptied, programs you haven't used in a long time that you might be able to uninstall without missing, etc. It won't do ANYTHING without you telling it to, and it'll give you an idea of the risks of each recommendation and how much disk space you'll gain back.
I did the disk cleanup but it only found 3 K of files to get rid of. I did those but I am still getting the disk full message, on start up.
I am also concerned that there will be a problem the next time I go to do a restore point.
Note: No trees were killed in the sending of this message, but a large number of electrons were terribly inconvenienced.