What is the best way to find out what files are sucking up so much space? My Windows directory lists its size as 15 GB but when I run a powershell script against it, it shows 40 GB. It seems 40 GB is about right because if I use that 40 GB and add it to the rest of my folders on C I get the 45 GB that is in use.

I checked volume shadow copy and that is disabled and there are no volume shadow files listed. My page file is limited to 3 GB. How can I find out what files are taking 25 GB of space?

Final Results: We ended up disabling UAC and then the Properties showed the true file size of the Windows folder, as shown in powershell script. This allowed us to drill down to the folder that had the issues... In our case it was in C:\Windows\System32\config\systemprofile\AppData\Local\VMware\VDM there were several large dump files, these were the mysterious unknown files in WinDirStat. It turns out our nessus scan is crashing a view service for a couple seconds so it creates a large dump file. So we will try this.. https://discussions.nessus.org/thread/5212