First of all we would like to let you know what the boot.ini file is, where it is located, what is its purpose and so on.
The boot.ini file is an initialization file used by the Microsoft operating systems and it is located in the root of the primary partition where the system is installed. The boot.ini file let’s you select the operating system you want your system to boot on your PC with an easy to use navigation.

Usually there are two sections in the boot.ini file. One is the [boot loader] and the other [operating systems]. In the [boot loader] section you can set the time for the boot menu to be displayed on your screen, usually in seconds. In this section you can find the default operating system which will be loaded automatically. This is needed as there might be more operating systems on the PC and one should be the default which loads automatically after the timeout period set in the [boot loader] section.

Here is a sample for the first section in the boot.ini file:

[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS

The second section is used to display and specify the locations of all the operating systems installed on the computer. Here is an example for this section with all the options:

[operating systems]
multi(0)disk(0)rdisk(1)partition(1)\WINDOWS="Microsoft Windows XP Professional" /fastdetect

How to edit and rebuild the boot.ini file? 

The boot.ini file is a hidden and prior anything you will have to set your system to show hidden files then make a backup of the boot.ini file.

There are two ways to edit the boot.ini file:

1.    Right click on My Computer icon on the Desktop then select Properties, Advanced tab, Startup and Recovery Settings then click the Edit button.

2.    Another more quicker way to edit the boot.ini file is to use the following command in Command Prompt: sysdm.cpl then hit Enter. You can do this by selecting Start then Run and type the command and OK, or in DOS Prompt use the same command.

How to modify the boot.ini file? 

The Windows XP Professional operating system comes with an executable file used to configure or modify the boot.ini file. All you have to do is to write the following command in DOS Prompt: bootcfg.

There are commands for adding, deleting, setting default OS and setting time out period.
Here is an example for adding an additional operating system like XP Home Edition: 

bootcfg /copy /e Windows XP Home Edition /ID#
There is a boot entry ID at the end of the command.
In case you want to delete an operating system from the boot.ini file, the command is:
bootcfg /delete /ID#

The command for setting the default operating system to be loaded is:
bootcfg /default /ID#

And finally here is the command for setting the time out (in seconds):
bootcfg /timeout#
Instead of the # sign, you will have to specify number of seconds (ex. 5, 30).

How to repair a damaged boot.ini file? 

You will have to run the Recovery Console for which you will need the Windows XP CD and boot from it then press R when asked.
After this you will have to enter OS load option. You will type /fastdetect which makes the detection automatic. Hit Enter Select the operating system then as soon as you arrive at the command prompt write the following command: bootcfg  /rebuild. There will be a few steps you’ll have to follow and you will have to press Y when the identification of the OS has been completed. You will have to enter the load identifier which can be Windows XP Home Edition or Windows XP Professional then hit Enter.

After this you will have to enter OS load option. You will type /fastdetect which makes the detection automatic. Hit Enter then restart your PC.

When troubleshooting a computer there are some other boot.ini switches which can be used like: /bootlog, /debug, /safeboot, /w95dos, /year just to mention a few.