Registry setting for Internet Explorer Protected Mode

Internet Explorer's Internet Options dialog has a setting called Protected Mode, which is a security mode that can be enabled or disabled for each zone. The Registry setting for it is stored with the rest of the Internet Options items, under the appropriate numerical subkey here:

HKCU\Software\Microsoft\Windows\CurrentVersion\Internet Settings\Zones

The numeric zone IDs are: 0 for Local Machine, 1 for Intranet, 2 for Trusted Sites, 3 for Internet, and 4 for Restricted sites.

A REG_DWORD value named 2500 controls Protected Mode. The entry is 0 if Protected Mode is enabled and 3 if Protected Mode is disabled for that zone.

Based on my Super User answer.