The free, easy way:
Use Windows Movie Maker, which should already be on your computer. The side effect of this method is that your AVI will become a WMV, which may or may not matter depending on who your target audience is.*
[li]Go to Start -> All Programs -> Windows Movie Maker.[/li][li]Import your video and drag it into the storyboard/timeline (the area at the bottom with a lot of blank squares).[/li][li]Right-click on the clip in the storyboard and choose “Video Effects”. You’ll be given a list of effects you can use, including various Rotate functions.[/li][li]When you’re done with the editing, go to File -> Save Movie File (CTRL-P) to export it back to a video file (until you do that, it’ll only be a Movie Maker project that nobody else can watch).[/li][/ol]
*The free, not-so-easy way that’ll preserve your file format:
VirtualDub. You’ll only need this if you’re not satisfied with the Windows Movie Maker output for some reason. Before I elaborate, I have to ask: How did you want to distribute the video once it’s rotated?