Why is the available space on the Windows 7 C disk getting smaller and smaller?

In general, Windows 7 will occupy about 20GB of hard disk space. During the use process, it is normal for the disk space of the system disk to become less and less.
C Where is the available space for the disk?
These spaces are used here:
Registry: User data and program data are continuously written to the registry, so the registry will become larger.
Patch Installation: When Windows Update auto-update is turned on, there will be a new patch release almost every month. Installing the patch requires extra space. (Patch replaces some files and backs up the original files.)
System Restore: System Restore automatically creates restore points and requires hard disk space.
Some places also use some disk space, not much to say here.
Can you free up some used space?
You can try these methods:
General Disk Cleanup
We can use the tools that come with the system to perform some regular disk cleaning. Improve performance by defragmenting your hard drive, using disk cleanup to delete junk files, and more.
Set the space usage limit for system restore
If the system restore function is turned on, the system will also automatically generate some image files, occupying the capacity of the C drive. We can set an upper limit for this part of the space based on the capacity of the hard disk. This step requires administrator privileges.
On the Start menu, right-click on “Computer” and select “Properties”.

Click on the “Advanced System Settings” on the left side of the window.

If the user account control window pops up, please allow to continue.

Click on “System Protection” tab. Click the “Configure” button.

Pull and pull the buoy to adjust the space usage limit, and finally click “OK> Save changes.

Turn off sleep, sleep plan
In addition, if the sleep hibernation function is turned on, a hidden system file hiberfil.sys will be generated in the root directory of the system. It takes up a certain amount of system space, generally the same size as computer memory. Therefore, if the system disk space is not large enough, this large file will become a burden. If the system disk is almost full, then turn off this feature.
Type “Power” in the search bar of the Start menu, then click “Power Options”.

Find the current power plan and click on the "Change Plan Settings" link.

Click “Change Advanced Power Settings”.

Find and expand the "Sleep" item, set "Sleep", "Hibernate" to "Never". Finally press “OK” to save your changes.

If you are using a Vista system, you can refer to Yi Baodian: Why does the space used by the C drive increase rapidly during the use of Vista?