What happens when I delete the entire Windows registry?

You can’t delete the root nodes. They don’t physically exist. You can, however, delete their contents, via Regedit (as opposed to reg).

Regedit hangs as soon as I try to delete HKLM\SYSTEM. After resetting the VM (because I’m lazy), I get the following screen (OS is Windows XP):

(“Windows could not start because the following file is missing or corrupt: \WINXP\system32\config\SYSTEM”)

So yes, deleting stuff from the registry will absolutely positively kill Windows. And unless you have a backup, restoring it is impossible.

Windows stores a lot of critical information in the registry, which is loaded (at least a part of it) during an early stage while booting. If you remove this information Windows will be unable to find and load critical system files and thus be unable to boot.

Such critical information is e.g. the list of device drivers needed for booting the system. This may include:

• disk drivers (floppies, hard disks, CDs, USB-devices, ...)
• bus drivers (IDE, SATA AHCI, ...)
• file system drivers (FAT, NTFS, ...)
• ...

However, by default Windows stores a backup copy of the registry. If you've enabled "System Restore" you can find such copies in the (hidden) System Volume Information Folder inside the root folder of the system partition. Additionally, even if you do not have "System Restore" enabled, Windows saves a backup copy of the registry in %WINDIR%\System32\config\RegBack\.

You can manually copy the backup files into %WINDIR%\System32\config\ to restore the registry data.