If all methods above fail, this is one last (more advanced) method to repair your Windows XP boot.ini file or to backup your files before formatting and reinstalling your Windows XP ;)
First you need to get a version of Backtrack (Live Boot version) and write it on a CD.
You can get backtrack here:
Once you have a Backtrack Live Boot CD/USB, insert it in the corrupt machine.
Boot Backtrack and login if necessary.
Open a kernel and type:
You should now be working in the mnt dir
Now you should see a lot of other directories, but you should select sda1. (This may be sda0 or other, depending on your system.)
So to go to the sda1 dir, type:
This command lists all your files in the working directory.
You should see your hard drive, with all the directories ;)
Now if you want to recover files from your hard drive, just open them with BackTrack and copy them to your USB or external HD.
Now if you want to edit your boot.ini file, you just open it in BT and adjust the content.
This is an example for Windows XP Home Edition (on partition 1 of your harddrive, if you have partitions)
multi(0)disk(0)rdisk(1)partition(1)\WINDOWS="Microsoft Windows XP Home Edition" /fastdetect
After you have edited your boot.ini, just save it and reboot Windows XP without BT.
I hope this was helpfull ;)