by default, CCleaner is set to clear Windows Defender's scan history (listed under Utilities in the Applications tab), at least on Windows 10 systems.
this isn't recommended, as it turns out that Windows Defender uses the latest entry in its scan history to determine whether or not to perform an automatic system scan when idle. if the scan history is cleared, automatic system scans will never occur. it will probably still automatically scan files as they're downloaded but it may send alerts to quickscan the system