You could create a still picture with your image editor of choice. I just tried Ulead PhotoImpact that I got almost free on eBay. I made an image 720 x 1440, filled it with black, used a particle effect filter and blurred it slightly. After dropping it into the overlay track, I used the Chroma Keyer filter to drop-out the black, and used the pan and zoom to pan from bottom to part way up, which looked right for my title length. It looked better by adding a .04 Gaussian Blur filter to it, also. You'll have to pre-render the clip for a decent preview. You could probably even get multiple layers and sizes of snow falling at different rates. Ulead has a free functional trial of their software if you don't have something else, but it's probably a big download. There's lots of tweaking of the particle effect filter available. Here's a link to the picture I used, if you want to check-out what may be possible, but I really didn't mess around too much. snow.jpg
There is a free program called Terranim8or which allows you to create different types of effects (e.g. water, clouds, and yes snow). Haven't tried it yet, but I have played with Anim8or on which Terranim8or is based.
Edit...
I found this web site that has a tutorial on making snow with Terranim8or. It can be found here.