|"If I delete the D, will that add the 7.81 back to C?"|
It's not as easy as that.
1st of all, download, install, & run CCleaner-Slim...both the cleaner & the registry scanner. Delete everything they find. Assuming you're not the type of person who regularly dumps temp files & accumulated internet garbage, this should free up several GB of space:
download CCleaner Slim
Then you have to start looking for useless garbage programs & files on your system. How much data is in your My Documents folder? Right click on it, click Properties, then click the General tab. What's listed for "size on disk"?
Go to Control Panel, Add or Remove Programs & look over the list. Are there any programs on there that you no longer use? get rid of them.
BTW, make sure to dump the recycle bin. Everything in it counts against the total drive space. It's a trash bin, not a storage bin.
Another thing you can do is adjust the amount of drive space used by System Restore. The default is 12%. Move the slider so that it's about 2GB, like shown here (scroll down to the 1st screenshot):