I have become a big fan of the web based screen casting tools, any OS as long as java is up to date. None of these require you to have usernames to record, and download to the computer, but they will if you want to keep the videos in the cloud.
I use Community Clips. Couldn't be easier...records in .wmv so it can be
opened in Movie Maker and edited. It is free. Very easy for students to use.