When the tracking feature is disabled by the user:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced:Start_TrackDocs=0x0 (REG_DWORD)
When the tracking feature is disabled through Group Policy:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer:NoRecentDocsHistory=0x1 (REG_DWORD)
regards, Martin