My desktop based on Windows is equipped with 180GB for C drive. I do not remember when, but suddenly I felt that the desktop becomes slow. After a quick examination, I realized that the C drive is full of files, and the remaining space is almost exhausted up to less than 10GB. So, I spent a few hours to delete unnecessary files and/or to move some to other storage. I managed to get the C drive back to have around 30GB. This is good enough, say, for Internet surf, but not for running Ubuntu on VirtualBox.
One day some idea hit my head to think of the existence of some utility to give us some statistics on the sizes of files and directories. After a quick search on Internet, I found this utility, WinDirStat. After installation and run, it immediately showed the sizes of directories and files in order. It was very easy to recognize what occupies the C drive most. After a few seconds, I was so excited to find out that there is a 45GB temporary file somewhere under C:\Users\MyUserName\AppData\Local\Temp\. This file has been left because a well-known domestic ZIP utility called AlZip didn't delete it after uncompressing it temporarily. After deleting this file, the C drive is now capable of holding almost 90GB, which is enough for running Ubuntu on VirtualBox without any noticeable lag. Great!
https://windirstat.info