Friendly reminder, after checking the file integrity in Steam properties, launch the game once, otherwise the installer WILL NOT FIND the game.




Make sure, you're logged in on the account with the administrator privilege, otherwise you won't be able to proceed.
Now, launch registry (simply put regedit in search bar).

Look for HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\electronic arts\Electronic Arts\Mass Effect (tm)\ergc. Swap the value from %CDKEY% to 5GUCXRFPQNBMTJBD3L5J.
Next, go to the local files of the Mass Effect game, look for installscript.vdf file, and edit it with notepad.

Swap the %CDKEY% to the same code as above, save file, and the installation should run just fine.
If the installer will ask for the key, just type the same code as before.



Everything should run just fine.

Cheers!