blurring movies

Hi,

I have a movie that I want to blur in some spots.
The reason is that I want to post it on the web and there is some personal stuff in it. So is there like a plugin for imovie that can blur it, or is there an external app that can do it instead?

Thanks

MacBook 1.83 GHz White Mac OS X (10.4.8) 512 MB, 60 GB