To convert FLV files to AVI/DivX files,I use Dr. DivX (but it does not work with FLV4 files):

http://labs.divx.com/DrDivX

For FLV4 files,use FLV Extract to separate the video (saved as avi with FLV codec) and the audio (saved as mp3):

http://moitah.net/#FLVExtract

Then use VirtualDubMod to convert to DivX or Xvid