1. Double-click to open the computer on the desktop. In the computer window interface, select disk C (Win7 system disk), press the right mouse button, and select properties in the right-click menu bar.

2. In the System (C:) property window, under the General tab, click Disk Cleanup.

3. After the disk cleaning is completed, check all the files to be deleted in the "System (C:) Disk Cleaning" window that pops up, and click "OK".

4. Click "Delete Files" in the pop-up prompt window to indicate that you want to permanently delete these files, and Win7 system will clean them up by itself.

Method 2: defragment the disk (disk C).

1. Click "Start", enter "Disk Defragmenter" in the start search box, and press Enter on the keyboard.

2. Click "Delete Settings (R)" in the pop-up prompt box "Disk Defragmenter has been planned by other programs".

3. Select the system (c:), and then click the analysis disk (a).

4. After disk analysis, still select system (C:), click "Disk Defragmentation (D)" and wait for Win7 system to complete automatically.

Method 3: uninstall the software that does not need to be installed on drive C.

1. Click Start → Control Panel → Uninstall Programs.

2. In the program and function window, find the software that has been installed on the C drive, but there is no mandatory requirement to install it on the C drive, select the corresponding software, press the right mouse button, and select "Uninstall" in the right-click menu bar.

3. Re-download the uninstalled software installer from the Internet, re-install it, and change the default installation address and part of the storage path to a disk other than disk C (Win7 system disk).

The last and most thorough method is to reinstall the system, completely empty drive C, and only empty the system.