The default video format for recording a video in Windows Media Center in Vista is .wtv only if you have TV Pack installed, also keep in mind that Microsoft has shifted from the old dvr-ms format to the new wtv format. In Windows 7 beta the default recording format is also wtv. There are many commercial software that support converting of wtv to other video format, but unfortunately such feature is not present in any free video converters.
So the best way to convert wtv to other video formats is to first convert it into dvr-ms and from there convert it to other video formats such as mpeg, avi, etc. This is a little tricky but an easy way to do it.
Note: You can also convert wtv files using MC-TVConverter.
To convert wtv to dvr-ms, there is a free command line tool available called ToDvrms. Download it and copy the executable file(todvrms.exe) to the same folder where the wtv video file is present.
Now enter the following command line,
todvrms.exe c:test.wtv c:test.dvr-ms
It will look like this in the Command Prompt,
where c is the destination of the video file, and ‘test’ is the name of the video file. This command line will convert test.wtv in c drive into test.dvr-ms.
You can also open the Graphical Interface(todvrmsgui.exe) and type the input and output destination there, but the command line method is more easier for novice users.
Now once you have converted the video to dvr-ms format, you can then go ahead and download DVRMSToolbox, it is a tool for converting dvr-ms into other video formats. Since it has a graphical interface, converting it would be easy here. Enjoy!