The windows XP installation disk is the safest and most reliable method of repair.
Look in this forum, fourth post from the top, for the directions to do a repair installation. This does not erase the personal files.
There is another way, depending on which file it is ex: command.com corrupt or missing error.
Go to Microsofts site and search missing or corrupt whatever file it is from another PC and print out the instructions it is a long and tedious process but it will get your system back up, it pretty much copies a file that is backed up by your system not you and you copy it to your current system.