I really need help. I have a upconverting VCR/DVD recorder that can play MPEG-4 movies, but everything I tried doesn’t work. Looking at the manual I noticed that “Playable Codec Format” is MP43. Does anybody know how I can convert movies that I have saved on my computer which are either AVI or MP4 (for PSP) to this MP43? I really need help..... 
Goddamn spammers! I deleted a few of their posts...
This is kind of late, but better late than never.
Use VirtualDub: it’s FREE and open source.
http://www.virtualdub.org
And for MP43 get this patch:
http://www.moviecodec.com/downloads/28d.html (old version of Microsoft’s MPEG-4 codecs that allow encoding in MP43 in AVI)
When you open your video file in VirtualDub, click video, compression, and pick “Microsoft MPEG-4 Video codec V3”. Tweak any settings you want by clicking on configure. Then click file, save as avi.
If you need plugins for VirtualDub, you can get some here
http://home.comcast.net/~fcchandler/index.html
or google for more.
P.S: If when opening in VirtualDub you get a VBR audio warning, use NanDub instead. Very similar to VirtualDub, also freeware, but supports VBR audio.