Behind the window sounds good, put a scrim upstage of the window. which will contain the snow flakes even if you use a fan to blow them about. I f you have a cyc or act drop beyond the window. With a tweek or a play about bit, you can change the lighting state, if it has to be snowing heavier.

I built a small, motorized "hopper" (I used a synchronous timing motor - very slow RPMs and completely silent - controlled from the tech booth) that metered out pulverized styrofoam (packing peanuts run through a blender - CAUTION!  It generates a pretty good mess) and positioned it up in the ceiling, directly over Paul Bratter's head as he lay on the sofa.  We used subdued (blue) lighting and the "snow" started coming down on his head, right on cue.  The styrofoam bits are so light they floated down perfectly.  You don't need much of it since that event is right at the end of a scene.

I believe Theatre FX (theatrefx.com) sells the bubble machines designed to make "snow".  They claim the bubble fluid they use does not make the stage slick or anything like that.