I have had a few .rtf folders appear on Windows Esplorer in C: drive. Also a .eula folder. They consist of a long sequence of lower-case letters and numbers. Hitting 'Delete' brings up "Access denied. Disk may be full or application may be in use.."
How do I get rid of these folders?

Try the following steps:-
In the Command window (Start/Run) type "cmd". A Command window will open. For the moment ignore it and goto the next step.
Hit ctrl+alt+del and then in the TaskManager Window that opens up click on the services tab and click on explorer.exe and choose end process. Then close this window.
In the still open command window type:-
"rmdir 'c:\documents and settings\<your foldername>'"
Note the quotes carefully in the line above- single quote followed by double quote marks. Repeat for each folder.
Restart your computer once you are done