If you can view the Video on your PC you should be able to Convert it useing pretty much any Video encoder but if you can not watch it on youyr PC then you will need to install the Correct Codec before you will be able to watch or convert the File ....
I have never heard of the MRV Format so it is most likely Proprietary ,meaning it probably can not be Watched or Converted unless you are useing Software Created by the Maker of this Format....
It could also be a Standard Video format but just have a Different extention so renameing the extention to MPG or AVI or WMV might get it working.....
You can use a Program Like "GSpot" to Find out the Info on this File like what codec is needed ECT....
Cheers