Thanks to Jane's E-learning tip of the day, I found out about Screen Toaster- it records you desktop and put it into a variable size video. check it out. will be a good tool for easily creating instructional demos. You can email the link or embed the finished video in blog, wiki or website.
But why would you want to create screen casts?
Well, take a look at the research, and blogs that supports short podcasts for lectures. here is just one to start.