First thing you need to do is a backup copy of all your important files. You can use an external HDD for that.
Then try running the repair , yes it is done by booting from the XP CD and choosing the repair option when it starts installing. If should ask you in the wizard if you want to repair an existing installation of Windows, at that point you select that option and on next page you select the one (and only) Win installation you already had; that should be recognized as such by the wizard and therefore displayed as an option.
Then follow the wizard instructions. Get back here if running into issues.