The main riff. I always make a mistake when attempting to play it along with the actual song. Didn't get in as much practice yesterday as I'd hoped. Another problem I have is the strings make a buzzing sound, so I was thinking maybe a change of strings is the way to go, as they seem quite rusted.
The main thing with power chords is keeping your fingers in the same position while shifting them from fret to fret. You just have to keep practicing and you'll get it =P
String buzz usually comes from not pushing down on the frets hard enough, but yeah if your strings are rusty you should probably change them...