Windows registry checker tool aka Scanreg.exe starts working once you start your computer. It creates a backup of system files and registry configuration information once daily. Files that Windows Registry Checker backs up include System.dat, User.dat, System.ini, and Win.ini.
What does Windows Registry Checker do is it automatically scans the system registry for any invalid entries when a computer is started. If invalid registry entries are detected, it automatically restores a previous day’s backup; all this is equivalent to running the scanreg /autorun command from a command prompt. If no backups are available, Windows Registry Checker tries to repair the registry. This is equivalent to running the scanreg /fix command from a command prompt. If the registry contains more than 500 KB of empty data blocks, Windows Registry Checker automatically optimizes it.
Windows Setup runs the Windows Registry Checker tool to verify the integrity of the existing registry before it performs an upgrade. If it detects registry damage, it tries to fix it automatically.
Scanregw.exe is protected-mode version of the Windows Registry Checker tool, which creates a backup of the system files and scans the registry for invalid entries. If invalid entries are detected, it refers to the real-mode version of the Windows Registry Checker tool (Scanreg.exe) for a resolution.
You can use Chily Registry Repair software for invalid entries cleanup and for backing up & restoring whole registry that get corrupt due to registry errors, virus attacks. It is the simplest and easiest tool that performs all functions related to Windows registry and works well when Windows Registry Checker tool fails to perform.