I need help...

[Stalker]

GOOGLE.COM
I need a [free] animation program that can take a few 5-10 frame looping animations and mask each one with a moving vector and layer them. Any OS, preferably Windows XP. Any ideas?
Basically, I need something for weather forecasts for our student-run TV station; I figure I can make a layer of clouds, a layer of rain, a layer of snowflakes, all at 50% opacity or so over a base image and then just animate a mask on them or animate an outline for them.
Help here...