WMV to MP4

[MrCleanit]

Anyone know of a free software I can download to convert a WMV file to a MP4 file that is trust worthy? I want to make a few kids movies to a MP4 and swap them over to my Wife's tablet so when we are out they can watch movies from it.

As you can see I'm at a lost when It comes to this stuff.

Any help would be appreciated.