1) In Disk Manager, see if there is more than one partition, or if the partition is smaller than 500 GB. Or right-click in Explorer and look at the Properties.
2) If it really is a 500 GB partition, and Properties shows over 400 GB used, try a utility named Windirstat. Run it from an Administrator login, and check the option to show hidden files. It does a good job of showing where space is used.