I shot some movie clips vertically on my digital camera. How do I rotate them horizontally to view them?

I bought a Sony Cyper-Shot Carl Zeiss digital camera a year ago, and just started making little movie clips. I made a few that were shot vertically, however, when they come up on Windows Media Player you have to look at them sideways. How do I rotate the ones I've already made so they can be viewed horizontally?

Right click on the clip on the timeline and select video effects. "Rotate 90" or "Rotate 270" should do it for you.