Get fraps and play the demos and use fraps to record the good bits, use windows movie maker(if you have window installed you have it) to edit and string them all together. Pm me if you need any more help, I don't know of any other free video editing software though...
If you use windows movie maker, be sure to save your final video in a resolution and bitrate high enough that the important details of your movie are obvious. Far too often, people use a format like wmv with very high compression, and they think everything looks fine...
... but that is because you're making the movie, and you know exactly what to look for. You already know what the chat console says and you know exactly when to look for that knife kill, others don't.
