Here is a central list of things to do, in order to repair and clean up a Windows 10/11 installation.

All commands must be run in a Administrator Terminal. Right-click on the Windows symbol and select Terminal (Admin) or Powershell (Admin).

Repair

DISM can be used to check and repair the Windows Component Store:

Dism /Online /Cleanup-Image /CheckHealth 
Dism /Online /Cleanup-Image /ScanHealth 
Dism /Online /Cleanup-Image /RestoreHealth 

SFC is used to check and repair Windows system files:
sfc /SCANNOW

Cleanup

Remove old updates using DISM: (You won’t be able to uninstall any previous updates after.

Dism.exe /online /Cleanup-Image /StartComponentCleanup /ResetBase

Then clean unused files on the disk:

cleanmgr /lowdisk

Antivirus

Windows has a built in antivirus called Defender that is active by default and does not need to be replaces.

If you are uncertain wether you might have caught something, download the Microsoft Safety Scanner and let it run a full scan (overnight)

Links

Further reading:

https://woshub.com/dism-cleanup-image-restorehealth

https://learn.microsoft.com/en-us/windows-hardware/manufacture/desktop/clean-up-the-winsxs-folder