Hi dmaggs,
Some of the space is used up by Windows itself, this can't be helped, but some of it is used for specialist recovery information that helps the OS self-heal and provide the ability to recover automatically if you have a bootup/OS issue. This can be removed and put on a USB stick instead, but don't lose this or you may have issues recovering the system in future.
One of the largest regular space takers can be the "System error memory dump files" - these will be created each time your system has some kind of fault. I suggest the first step is to clean down all unnecessary files, such as those created temporarily whilst updates are applied, and these memory dump files:
- Go to My Computer
- Right click on the C: drive and choose Properties
- Select "Disk Clean-up"
- Select "Clean up system files" (this will take a while to calculate)
- Review the list of items, ensuring ticks in the relevant boxes and run the clean up (click OK)
- Check your new Free Disk Space amount to confirm and increase
- Repeat the process once a month
You can then search your C: drive for other large files/folders using a trial version of a great tool called "Tree Size" http://www.jam-software.com/freeware/index.shtml
If you do want to remove the recovery information, follow these instructions: http://www.networkworld.com/article/2164909/windows/windows-8--how-to-free-up-disk-space-on-microsoft-surface-tablets.html