There are a number of "pay" programs such as GameCam and Fraps that capture your screen. I recorded a short demo recently and the video clip resulting from about 10 seconds was 20 MB, so make sure you have space on your hard drive. I used a free, open source program called CamStudio (http://camstudio.org).

Basically you have to play your demo in TF2 by entering "playdemo name" in console. Then when the demo starts playing, you can type either "demoui" or "demoui2" in console to get playback controls. Find the spot where you want to record, pause it, then start up your screen capture software and resume playback. After you finish recording, you can edit the video file in Adobe Premier or Windows Movie Maker.

k thanks.