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An answer to PParks question will be very helpful.
You may find this program helpful.
TreeSizeFree: JAM Software - Windows Freeware
One cause of what you are seeing is malware of the worst kind. A normal online anti-malware program will not be able to remove the malware.
this why you need to run WDO, Windows Defender Offline, Link in my signature.
WDO will run a quick scan by default. Be sure to run a FULL scan over all drives.