When attempting to install QuickBooks Desktop, the following error occurs:
Error 1638: Another version of this product is already installed...
To resolve:
  1. Check to see if QuickBooks Desktop is already installed.
  2. Go to Start > Control Panel > Add/Remove Programs.
  3. If the version of QuickBooks Desktop you are installing is listed here, it is currently installed.
  4. If QuickBooks Desktop is installed already, you can open the program and use it: 
    1. Go to Start > All Programs > QuickBooks
    2. Click the version of QuickBooks Desktop you wish to use
  5. If you can not open the program, Repair QuickBooks.
  6. If repair is unsuccessful, perform a Reinstall using a clean install.
  7. If unable to uninstall the program, look up the error message you are receiving when attempting to uninstall on our QuickBooks Support website (US, Canada or UK).
  8. If all of the steps above fail to uninstall the program, use the Program Install and Uninstall troubleshooter to remove QuickBooks.
  9. If none of the above steps resolve the issue, try installing QuickBooks to a different directory folder.