You can go get TREESIZE to look around but that's a tiny drive today. You would have to know your OS very well and turn off and back on system restore to keep it tidy for starters.
Did you look in the System Volume Information folder too?
I have a 115 GB Hard Disk as a system drive running on windows server, but it says only 4 GB left, I do not have much program installed on the server.
where should I started to delete the file or where should I started to check, which files takes up lots of space.
I know by simply adding up the number by right click on files property and compare the sums to the drive usage pie chart is wrong, but what else should I do?

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