There are a variety of free options out there. FCPS computers are equipped with SnagIt, and I use it frequently. Simply click on your start menu and search for SnagIt. Open SnagIt 12. Then, open the Toggle Capture Window. Select video from the options and begin recording. There's a video demonstration, below.
This is a free tool available on the web and you can record and download a screencast that is up to 15 minutes long. It's very user-friendly. Check out the site and demo here:
If you want to get a little fancier, EduCreations allows you to create white-board style lessons, like you see in Khan Academy videos. There is an iPad app and it will also work on your laptop.
Educreations is an interactive whiteboard and screencasting tool. It allows you to annotate, animate, and narrate explanations of concepts share instantly with students.