Instructions
 
Click "Start," "Programs," "Accessories" and then "Notepad." 
Type commands into the text file, such as "-windowed" or "-minspecaud." They all should be a on a single line. Separate them with a single space. A full list of commands is available in the reference section. 
Press "File" and then "Save." Navigate to your game directory. By default, it installs to "/Program Files/Rockstar Games/Grand Theft Auto IV." 
Name the file "commandline.txt" -- omitting the quotation marks. 
Click "Save." 
Start "Grand Theft Auto IV." It will read the commands in your script automatically, changing the game's values.