Please I need Help With Windows Installer!!!!!!!

Ok now this is a bit long so bear with me. I have been using Windows Installer for quite a wile and one day it just up and quit. I have been trying to Install a game on my computer call Age of Mythology. After a full setup and installation with no problems I open the game to get an error message "msxml 4.0 must be installed to continue" So I opened the destination directory and then opened the msmxl file. All of these were MSI files, so I right clicked and clicked install. This just brought up more errors that say "The Windows Installer service could not be accesed. This can occur if you are running Windows in Safe mode (Which I am not), or if the Windows Installer is not correctly installed." Believe me I have tried everything I could think of. I went to microsoft for help and msxml for help but both things help little to none. I cannot reinstall my operating system because I lost WAY to many valuable items last time. I would really apreciate an answer concerning this.