I spent ages trying to answer this question. There are a number of solutions out there but here is the one I would recommend.

Stage 1 - Convert WTV file to DVR-MS

In Windows file explorer right click on the WTV file and select "Convert to .dvr-ms Format". This is will convert the WTV format to the slightly older DVR-MS format as used in previous versions of Windows Media Center.

Stage 2 - Convert to MPEG2

To convert to a standard MPEG2 file you can use DVRMSToolbox available from andy vt's tools & blog. Use the DVRMStoMPEGGUI utility. Set the profile to "Convert DVR-MS file to MPEG".

Stage 3 - Edit/Trim & Export

Now you've got an MPEG2 file you can use any MPEG2 compliant tool to edit or trim out sections you don't want.

I use Womble MPEG Video Wizard because it allows frame accurate editing of MPEG video. Once you have made the changes you can export a fresh MPEG2 file without re-encoding. This means there is no loss of quality.