I think you can find a dozen MKV-AVI converters on the internet :S.
Use MKVExtractGUI to extract video, audio and subtitle from MKV files.
Then, you can multiplex (join) all those elements together with VirtualDubMOD.
MKVExtractGUI:
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http://corecodec.org/projects/mkvextractgui/
VirtualDubMOD:
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http://virtualdubmod.sourceforge.net/
After installing MKVtoolnix, unzip MKVExtractGUI to the same folder where MKVToolnix is installed to.
Taken from its site:
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MKVtoolnix will evolve into a set of tools to create, alter and inspect Matroska files(mkv). These are command line tools but mkvmerge GUI is included (mmg.exe). Use MKVExtractGUI to extract/demultiplex mkv video and audio files.
Download MKVtoolnix:
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Unicode enabled (Windows NT, 2000, XP or newer):
http://www.bunkus.org/videotools/mkvtoo ... -setup.exe
non-Unicode (Windows 95, 98 or ME):
http://www.bunkus.org/videotools/mkvtoo ... -setup.exe
I took the above from this thread:
http://www.moviecodec.com/topics/13049p1.html 