conversion from mpeg4 to avi using mplayer
 

please can you tell me how to convert from mpeg4 to avi using mencoder.

Something like this:

mencoder -ovc lavc -oac mp3lame -forceidx infile.mpg -o outfile.avi

If you want to scale as well:

-vf scale -zoom -xy 512

man mplayer for all options.