This article explains how to easily repair or replace the boot.ini file in Windows XP.
How to Repair or Replace Boot.ini in Windows XP | TechwithGuru
Repairing or replacing the boot.ini file usually takes less than 10 minutes, but the total time can be much longer if you need a Windows XP CD.
Go to the Windows XP Recovery Console. The Recovery Console is an advanced diagnostic mode of Windows XP, with special tools that allow you to repair the boot.ini file.
When you reach the command line (see step 6 in the link above), type the following command and press Enter.