As CaptainMARC says, render your animation to image file. Poser will render each frame as a separate image, and I believe .png is the default.

You can use a program like VirtualDub to then create an avi file from your renders. The advantage to this is that you can do postwork on your videos in Photoshop, GIMP, or After Effects. And you don't get the pixelization that rendering to .avi gives you in Poser.

Here is a screenshot of how to set it up in Poser. The red highlight is the output file type. Orange is the render dimensions. This is changed from default in the render tab under render dimensions.

If I had a nickle for ever time a woman told me to get lost, I could buy Manhattan.