@grumpyfish gave a good answer. To clarify a bit:
It’s not the window that’s leaking (most likely, although that’s also possible; you probably would have noticed a leaky window during a rainstorm in the fall, so it’s unlikely that “a leaky window” is the problem). What you’re seeing is a symptom of the problem on the roof.
The roof becomes a problem because (as @grumpyfish said) snow is melting on the roof. It does that because your attic insulation is insufficient to keep the large volume of snow from melting at the surface of the roof and running down to the eaves, where it re-freezes because there’s no warm surface or air there to prevent that. That builds up and eventually causes the water trying to run from the roof to pool—even on the sloping roof. All that you’re seeing is the symptom as water has worked its way under the shingles, through the roof skin, down the walls, and into the living area. (It’s also leaking inside the wall, which is another area you’ll want to examine when you can. You don’t want to just fix “problems you can see” and ignore others just because you can’t see them.)
The snow melt on your roof forms a vicious cycle: snow melts, water runs down until it hits the existing barrier of ice at the eaves… and builds higher. That means your pool of snow melt creeps higher on the roof. Most building codes call for special roofing materials at the eaves to prevent the “normal” ice buildup to within 2’ or so of the edge of the roof. When you get massive snowfall, the ice dam prevention is overcome as that water backs up on the roof and under the (good) shingles.
There is another remedy, but it’s not safe for you to do it now (keep it in mind for next time you get a significant snowfall): clear some of the snow from your roof, especially at the eaves. With ice on the edge of your roof now this is NOT RECOMMENDED. Don’t try to climb on your roof now unless you have excellent insurance, paid-up life insurance… and a death wish. Also, don’t attempt to melt the ice with flame sources. People who attempt this are often known as “fire victims” and “homeless”.