This post is addressing something that more and more teachers are beginning to use to instruct in a 24x7 scenario. Screencasting is taking video of a computer screen while someone narrates what they are doing.
There are many options for this type of video-recording, and while I will address two of them, I will also post a link here to an article which covers others.
When I am on my school PC, I prefer to use Smart Recorder. This tool comes with Smart brand IWBs, and is pretty user friendly. You click record, and when you're done and click stop, you tell it where you want the video to be saved. It is a pretty smooth program if you have it.
If you lack that, but don't want to pay for a professional capture software (which cost anywhere from $20 to $700), then you can use screencast-o-matic. This software will allow you to record up to 15 minutes of video instruction with sound and host it online or download the finished file. It is a pretty smooth tool, but the only downside is that there is a watermark on the finished video for the free version. It is a web app, though, so nothing needs to be installed.
Here is a list of other tools put together by someone other than me.