I'm guessing you left the default template unchanged. On the render screen once you've selected a template you can click on the "Custom" button to change it.

On the first tab, up the "Video Rendering Quality" to "Best". The default WMV template uses quality-based VBR which is set to 90% (that's the VBR 90). On the video tab, first try upping the quality to 100%. If that doesn't look any better, you can try switching the mode to something else, like "Bit-rate VBR" or "CBR (2-pass)".

It would be a good idea to save these settings as a template in their own right, so that once you've nailed it, you can re-use your hard work.

Also make sure that your project settings (video tab) has an interlace method set. Your video is low-motion, so probably "Blend Fields" would be ok.

EDIT: Meant to say that if you change the encoding to a bit rate-based mode then you will have to supply a bit-rate. Bit-rates as ever are a balance between size and quality, so you may have to play around with it. With the default template unchanged, the target bit-rate appears to be 500 kbps, looking at the "Bit Rate" tab - so that's a guideline for what doesn't work.