|I don't know about a 'line of code missing' error but if it's 'missing or corrupted' here's some info:|
That assumes it's not booting into windows and you need to start with the xp cd. Then you copy files from the windows\repair folder into the windows\system32\config folder.
If you only have a USB port for external drive support you really ought to get an external CD or DVD drive. Otherwise you'll need to create a bootable pendrive or whatever USB drive you use and copy to it those files from another xp computer. Then boot your junker with it and copy the files to their correct location.
If by chance it is booting into windows try doing a system restore to an earlier date.