In Control Panel \ System \ Startup/Shutdown, you can set the default instance and the default timeout. To add/delete or change entries, you must edit C:\BOOT.INI. This is a Read-only, Hidden, System file so in a command prompt you must type
attrib -r -h -s c:\boot.ini
Now you can edit the file in any text editor such as notepad. You can delete unwanted entries (don't delete the "VGA mode" entry for an active instance), change the text (from "Windows NT Server Version 4.00" to "MyCompany Server") or add a missing entry. When adding entries, you must understand the ARC path:
where Z is the disk number (starting at 0) and W is the partion on that disk (starting at 1). If you look at the "Target Device" entry in %windir%\repair\setup.log you will see the "Z" and "W" entries for your booted instance of NT. The "X" and "Y" entries for "SCSI" are more complicated and are beyond the scope of this "Tip". For addition information, see KB article Q102873.
Don't forget to set the attributes back (attrib +r +h +s c:\boot.ini).
For information on boot.ini switches, see tip 515.