To compress the video, download and install Divx or Xvid.  These are what I use.  There are probably other codecs out there that I don't know about.

1.  Load your video in VirtualDub

2.  Choose Video->Compression.  Select the Divx or Xvid codec.

3.  Under the Video menu, make sure Full Processing Mode is selected.

4.  Choose File->Save as AVI to save out to a compressed video.

I also like to save my animations out to an image sequence and just drag and drop the first image in the sequence (e.g. "image_000.png") into VirtualDub and the rest of the images in the sequence will be automatically loaded.  Then under Video->Frame Rate, I choose "Change frame rate to (fps)" and specify the frame rate of the animation set in Poser and I choose "Process all frames" in the Frame rate conversion section of the dialog box and then I select the compression codec and "Save as AVI".

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