Clean up the upgrade residue of Win 10. Turn on this computer, right-click drive C, select Properties, and then click Disk Cleanup in the properties dialog box. Click the "Clean System Files" button at the bottom of the dialog box. In the newly opened dialog box, find and select the "Previous Windows Installation" option, select the temporary file option at the same time, and click OK. At this time, you will be prompted, "If you clean up the previous Windows installation or temporary installation files, you can't restore your computer to the previous version of Windows." Just select "Yes". In this way, Windows.old and temporary files will disappear from the C drive, which can effectively release the space of the C drive.

Transfer virtual memory to another drive letter. Virtual memory is a way for Windows to allocate more physical memory for applications. Usually, the virtual memory file defaults to drive C, which may be one of the reasons for the lack of space on drive C. By transferring the virtual memory to other drive letters, the space on drive C can be effectively released.

The above steps can help you solve the problem of insufficient space on drive C. If it still cannot be solved, it is recommended to consult professional technical support or format drive C.