Leawo Free AVI Converter is a simple tool that will quickly convert your movies to the AVI format.
The program can handle a lengthy list of input file formats: FLV, WMV, ASF, WMA, AVI, RMVB, RM, RA, MOV, QT, MP4, M4V, DV, 3GP, 3G2, DAT, MPG, MPEG, MP3, VOB, FLV, MKV, MKA,MTS, M2TS and F4V.
You're then able to choose exactly what sort of output file you'd like. You could just select a particular AVI codec (XviD, H.264/MPEG-4, DV or lossless), for instance. There are also options to create files optimised for mobile devices, which also resize the original clip. You can even choose to simply strip the soundtrack from the source video, and save it as an audio file in MP3, WMA, AC3 and other formats, and then one click on the Convert button will transcode the movie for you.
Please note, the program interface includes many buttons and options which only work in the commercial version. This can be annoying at first, as you're forever clicking on something and seeing the same "if you want to use this feature, please purchase..." message pop up. You'll soon learn which features you can use, though, and if your conversion needs are simple then they'll probably be enough.
Other transcoding tools are more powerful, but Free AVI Converter's ease of use and good input file support mean it may be worth a look