Weird boxes on screen after using Windows Movie Maker to e..

wmv files that I make with Windows Movie Maker have a small 3x3 grid of
white boxes in the upper right corner of the screen.

This phenomena only happens under very specific conditions. It happens
when I:

1) Download a .tivo file from my TiVo to my PC
2) Convert the file to .mpg using graphedit and the Moonlight M17 muxer
and Moonlight-Elecard MPEG Demultiplexer filters
3) Edit the resulting .mpg in Windows Movie Maker
4) Use the "Save Movie File..." command to create a WMV file.

The boxes do not appear when I play back the .tivo or .mpg files from
the first 2 steps, and they do not appear when I perform steps 3 and 4
on a file from a source other than my TiVo.

Any idea what this is (some kind of TiVo secruity watermark perhaps?)
and how I can get rid of it?