There's two different options I'm going to suggest for recording videos in DBO. The first is
Fraps, but Fraps isn't free. There is a trial version, but you can only record 30 second long clips.
Fraps' user interface (UI) is pretty self explanatory so I'm not really going to talk about it. If you can get Fraps I suggest you do, it's probably the best recorder out there and is programmed specifically for recording gameplay. Most importantly; it can record sound in Windows Vista without any additional programs.
The free alternative to Fraps that I currently recommend is
CamStudio. Like Fraps, CamStudio's UI is self explanatory. The first thing you're going to want to do though is change the recording region to full screen "Region > Full Screen". To enable sound, if you're running Windows XP, go to "Option > Record audio from speakers". For Vista users it gets a bit more complicated, but try selecting "Option > Record audio from speakers" first. If you get an error: follow the steps below. If not: you're finished.
Steps to enable sound for CamStudio in Windows Vista
1. Download and install
Virtual Audio Cable (VAC).
2. Go to "Control Panel > Sound > Playback", select "Line 1", and set it to the default.
3. Open "Audio Repeater" (It was installed with VAC).
4. For "Wave in" select "Line 1" and for "Wave out" select your speakers and click "Start".
5. Go back to CamStudio and select "Options > Audio Options > Audio Options for Microphone".
6. For "Audio Capture Device" select "Line 1".
7. Select "Option > Record audio from microphone" and you're done.
Note : Doing this will disable sound from coming out of your speakers if you don't have the audio repeater opened and running. To re enable your speaks without the audio repeater go to "Control Panel > Sound > Playback", select your speakers, and set them to the default. You're now fine to close the audio repeater. You must set "Line 1" back to the default and open up the audio repeater if you want to record sound again.
There's probably other free alternatives out there, but this is the one I found that was easiest to use and didn't brand the video. If you use any other recording software and the sound doesn't work while your on Vista you'll have to go through pretty much the same steps listed above to enable it.
Note: If you're planning on recording videos during the beta start getting everything set up now and test it out so you'll have everything ready. You can modify the quality of the video in "Options > Video Options" if you need to. Also, by default the hotkey to start recording is F8 and to stop is F9.