http://www.zentao.com/guitar
http://www.digevent.com/events/consumer/music/guitar_mania/index.asp
The first site is a very well ordered basic course in guitar playing, reading music and tab. It will get you playing riffs in a few weeks.
The digevent site is a compliation of video lessons. You will see the riff played both up to speed, and measure by measure slowly. There are tabs included that you can download to practice with. Lots of different techniques are covered.