Cannot convert video files with mtv converter, using windows vista

Use another video converter, I'd recommend asoftech video converter as it helped me many time
http://www.asoftech.com/video-converter/

Try to use Leawo MKV Converter instead which you had used.

Home page: http://www.leawo.com/mkv-converter/

Leawo MKV Converter lets you make the most of MKV videos by converting MKV video to any popular formats, for example, MKV to AVI, MKV to MP4, MKV to DVD, etc.

Here is a MOV to MP4 converter. I have used it to convert between other formats too. You can have atry.

Leawo Video Converter could help you to do that. The conversion steps to convert wtv video are similar to the steps to convert mov vidoe in this article. https://www.leawo.org/tutorial/how-to-convert-mov-to-mp4-with-free-mov-to-mp4-converter.html

Maybe you can change another video converter. I'm currently using Free Online Video Converter that can do format conversion on webpage directly. It works finely on my computer, so I think it would be helpful to you as well.

This problem happened to me before, I switched to this all-in-one media converter, it worked.
http://www.reviewstown.com/ultimate-dvd-video-converter.html

Try another video converter! some converter may have limits, say, it only supports some format while other format cannot be its input format. there is a big possibility that the video you are using is such one.
as a all in one video converter, i recommend you the video converter ultimate, it supports various format, such as M4V, AVI, MP4, 3GP,, MOV, FLV, MKV, and so on, just have a try and your problem will be fixed easily!

Try freeware Handbrake. It works. Personally I convert flash videos to other formats with this swf to avi converter.

MTV converter just only convert MTV format video.
I recommen Enolsoft video converter(for windows).which can not only convert MTV format,but also convert between all common video format.

You can try this Cucusoft Video Converter Ultimate , which converts videos to play on almost any portable device including iPad, iPod, iPhone, Zune, PSP, video capable MP3 players, video capable mobile phones, and Pocket P.C., etc . Meanwhile, it can also support output almost any audio/video format including Video format(H264, MP4, WMV, AVI, MOV, RM, 3GP, flv, SWF, mpeg-1, mpeg-2, VideoCD, SuperVideoCD, DVD, etc.) and Audio format(AC3, AAC, MP2, MP3, MP4, RA, WMA, etc.)

Try to use other free video converter program.

Tenorshare Video Converter supports all kinds of video and audio formats conversion. the speed and quality of conversion are excellent.

The video formats it supports including:
MPG, MPEG, MPEG 2, VOB, DAT, MP4, M4V, TS, RM, RMVB, WMV, ASF, MKV, AVI, 3GP, 3G2, FLV, SWF, MPV, MOD, TOD, QT, MOV, DV, DIF, MJPG, MJPEG, HD TS, HD MTS, HD M2TS, HD MPG, HD MPEG, HD MP4, HD WMV, QuickTime HD MOV, HD H.264, HD RM.