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Click on the System Protection link on the left-hand side:
Once you’ve done so, the steps are different depending on your OS.
Disable on Windows 7
Click the Configure button below the list of drives:
Now simply click the radio button to disable System Protection. (Note again that this is probably a bad idea).
That should be all you need to do. Then, check the C drive space.
I would suggest you do the following too, if the issue persists.
Go to the link given below, download and install Ccleaner.
Install it and run it. After launching, click Analyze and then, run cleaner. After that, click Registry from the left pane of Ccleaner. Then, Scan for issues and Fix selected issues. There is no need to backup the registry.