Windows 7 Tip: 
Hitrec at VideoHelp forum introduces a software called RivaFLVencoder from which you can convert .flv files (Flash Video) into avi and mpg files. This one is the very useful software for those who download videos from Google videos or Youtube.
With this conversion, you can play google videos in many systems, such as iPod Video. One catch: Most of the time, audio can be transcoded to the target format. However sometimes audio codec cannot not be transcoded, you can follow this instruction to mux that back into the target file:
It’s possible that the .flv contains an audiocodec that will not be transcoded. A solution for that is to play the original .flv and record the audio with Audacity (for instance) and to mux that recording with the .avi or .mpg you get from Riva…..
Read More: How to convert .flv (flash video) to .avi or .mpg
I used that, when i convert flv to avi, it can’t give good video and audio synv output, I have tried with different encoders and only moyea gives the best video and audio output and can convert full video keeping file size low
http://www.flvsoft.com/flv_to_video/