When you log on, multiple windows may open. When this occurs, one window opens for each file or folder in the root folder on the hard disk. This occurs if there is no data value in the following key. This value points to the folder from which the Startup group items for the All Users profile are run. If not present, NT assumes that all files and folders in the root folder must be opened.

Hive: HKEY_LOCAL_MACHINE
Key: SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\
Name: User Shell Folders
Type: REG_EXAND_SZ
Value: %SystemRoot%\Profiles\All Users\Start Menu\Programs\Startup