The only working way at the moment is to convert the wtv video to dvr-ms and from there to another format. That’s not really practicable but it probably is only a matter of time until additional video converters add support for the wtv format. The command line tool ToDvrms can convert wtv files to the dvr-ms format.
The easiest way to convert the wtv files is by placing the todvrms.exe converter in the same folder as the wtv file to be converted. The command is the following:
todvrms.exe c:\test.wtv c:\test.dvr-ms
This will convert the test.wtv file to the dvr-ms format. The developer provides access to another tool that can be used to convert dvr-ms videos to another video format. The software DVRMSToolbox comes with a graphical user interface and can be used to convert the dvr-ms videos.
http://www.ghacks.net/2009/01/18/converting-wtv-files/