While our team is lucky enough to have 16 Power Mac G5's and Final Cut Pro as a result of the video courses we offer (and as Sam mentioned), iMovie is an excellent resource to fall back on (WMM really doesn't count) for video editing and lets you do most things that anyone really needs.
For Windows, I would recommend buying Adobe Premiere Elements for about $60 after rebate. It does an amazing amount of stuff, and having used virtually every piece of editing software there is, I feel it is second only to the actual Adobe Premiere Pro.
Here is an excellent comparison of many software packages.
As for Open Source/Free alternatives, take a look at
Kino, and
Cinelerra.