The best answer is to use a different app. Try Macrium Reflect free - it is much faster, more flexible, etc.
Macrium Reflect FREE Edition - Information and download
You do need to backup the 100mb system partition as well as C. If using Macrium, in Disk Management , rt click the 100mb partition and give it a drive letter. Tick both when selecting what to back up. It won't force you to backup D as well.
If you still want to use Windows to do it - try making sure nothing to do with Office is running. Look in Task Manager and stop it.
See if that helps.