just another tip, for those that may not be aware of it. If you have an ATI 4600 or 4800 series GPU, the AVIVO video converter I just downloaded, and it appears to support .wtv files to mp4 conversion with a very simple interface.
When I ran it it was using all my CPU power so not sure how much it was using my Radeon 4870, but an hour long HD video took about 20-30 minutes to convert which isn't bad. This may be the fastest way to convert videos if it is using the GPU. Ill have to do more testing.
ATi avivo video converter is available here for for free if you use an ATI card -
http://support.amd.com/us/gpudownload/windows/avivo/Pages/avivovista-download.aspx though they make you answer a few questions before downloading. Or use this link
http://game.amd.com/us-en/drivers_catalyst.aspx and select video card drivers > optional downloads.