If you have the XP recovery or installation disk you can boot with that to the recovery console and manually expand the file needed from the I386 folder's cab files found there. There are a few articles with insstructions for entering the manual command at the command line once arriving at the recovery sonsoler's prompt.
Insert the Windows XP bootable CD into the computer.
When prompted to press any key to boot from the CD, press any key.
Once in the Windows XP setup menu press the "R" key to repair Windows.
Log into your Windows installation by pressing the "1" key and pressing enter.
You will then be prompted for your administrator password, enter that password.
Copy the below two files to the root directory of the primary hard disk. In the below example we are copying these files from the CD-ROM drive letter "E". This letter may be different on your computer.
Once both of these files have been successfully copied, remove the CD from the computer and reboot.
What is meant there is to enter those commands while arriving at the recovery console's command prompt not browsing the XP disk itself since those files are compressed into the cab type archive files similar to what is seen with zip or rar files. The commands instruction those files to be extracted from the compressed one or more and placed on the root of the destination drive.
The method there does work quite well in fact. While trying to clean up a drive in one olser case here the ntldr was accidently deleted and saw the missing error. The simple extraction of a fresh copy of the file was all that was needed to see Windows load normally again.
Rebuilding the boot.ini file may not be necessary while it won't hurt anything either. That will depend mainly on what happened to the original boot loader file and why. If the boot sector saw files corrupted you could end up needing a repair install of Windows itself if no worm or boot sector type virus is found. If you do see Windows running fast again run a good sweep as a precaution with a few antivirus tools, adware/spyware removers.