The photo above is what I packed for a two-month trip, which included -10C temperatures in Prague in December and snow in Paris. The trick to traveling light in cold weather is layering. I wore my summer weight trousers and shirt, added an extra T-Shirt underneath, a merino midweight top, over and a waterproof jacket. Under it all I wore merino underwear and socks.
Merino isn't cheap, but it lasts many years, and compared to cheaper artifical fibres, it doesn't smell or itch when wet. And it holds it shape - that's why its gone from sportswear to high street fashion in only a few years.
I bought cheap gloves and hat on arrival, and threw them away as I boarded the flight out.
Everything I took fitted either in the blue pack above, or I was wearing it! The pack weighed around 8kg including a 10" netbook and other electronics, but with me wearing the shoes and jacket.