What do you want to do? For what it is worth you can dub something with audiodub on avisynth (
http://avisynth.org/mediawiki/Main_Page ) which is free and I can assure you that will work. It also has some features useful for animation.
On the other hand
do you want to make little animated GIF files?
I got used to Beneton Movie GIF:
http://www.snapfiles.com/get/benetonmovie.html in that case
Do you want to do "machinima"?
In this case get a copy of fraps (not free but very good) or Taksi (free but lacks audio support last I checked) and a game or three of your choosing.
Do you want custom 3d stuff?
I suggest modding/hacking a game and going the machinima route first but X3D/vrml with a backend using something like terragen
http://www.planetside.co.uk/terragen/ is also possible. Good news is frame rates need not be that high (you can do a demo run) and you can get away with a relatively low end machine.
Do you want stop-motion?
Can be made using a variety of sources but some of the more advanced filters for avisynth (mvtools is what I am looking at but some of the inpainting stuff is well worth a look).
Do you want 2d stuff?
I am not so familiar with this and there are hundreds of variants (how many cartoons have you seen?) but you can use some of the tools already mentioned. You can sometimes butcher a 3d tool into a 2d one as well which makes for some interesting viewing (especially for comedy).
Either way though I suggest you get mightly good at video editing, avisynth (linked above) is my tool of choice and is probably the most powerful non linear editor going.
