I would like to free up or see if I can transfer files from c to d drive and still have the system work as usual
Hi and Welcome!
you can easily move your “My Documents” folder to the other drive: http://support.microsoft.com/kb/310147
If you are talking about data files, not system or programs, you can just drag them to a new folder(s) created on D drive.
After that go to Properties, usually in Tools, of each program those files belong to and change a file location to your new folder. Than once you open the program and try to look for the file it will open in that folder, otherwise you have to look for it every time.