Microsoft Knowledge Base Article 311578 contains the following summary:
This step-by-step article describes how to edit the Boot.ini file in a Windows 2000 environment. NTLDR displays the bootstrap loader screen, where you can select an operating system to start. This screen is based upon the information in the Boot.ini file. If you do not select an entry before the counter reaches zero, NTLDR loads the operating system that is specified by the default parameter in the Boot.ini file. Windows 2000 Setup places the Boot.ini file in the active partition. NTLDR uses information in the Boot.ini file to display the bootstrap loader screen from which you select the operating system.
You should back up the Boot.ini file before you edit it. The first tasks include modifying your folder options so you can view hidden files, and then backing up the Boot.ini file.