I want a piece of software that will constantly record what I'm doing on my desktop, discarding footage that's over [30] seconds old. Its recording would be a rolling one.

The idea is that I can somehow hit a button and see "what just happened".

I don't want to have to babysit it. That is, I don't want a piece of software designed for screencasting (which I'm not trying to do). My bias against that is based on my (maybe incorrect?) assumption that I'd regularly have to start/stop the recording throughout the day.

The idea is that this piece of software would consume fewer resources (than a screencast recorder) on my box, as it's only keeping a very limited amount of footage in memory (and low quality would even be acceptable), because it's discarding frames fairly quickly after they're captured.

Where can I find a piece of software with features like this?