The files on the WMC box.... can you try right-clicking on them and selecting "Convert to .dvr-ms Format"?

This will create a copy of the WTV file in the same directory, but in the WMC format that prior releases used. It's a bit more accepted than WTV at this point. For example, Handbrake will work with .dvr-ms but not .wtv. So, it may work for your purposes. If it does, I believe there are programs out there to automate the conversion.