In reply to Fredt:
We've had temperatures of -25oC in the valley in January, February, March before, so if weather is bad, the roads can be terrible. Main roads are usually kept clear as the snow falls and they are pretty damned efficient, but depending on where you are living, you might not see a snowplow until hours after new snow, and if you are up in the arse-end of nowhere, it might even be the day after.
4WD isn't necessary, but snow tires or chains absolutely are, and there are police checks up and down the valley to make sure you are kitted out. If you don't want to fork out for snow tires then chains will do of course, and we have had winters recently where you'd only need them a handful of times in the few months you'll be here, but if it is a bumper snow year with regular dumps down to valley level, as some people (the same people as always...) are predicting, then putting chains on and off will get pretty old pretty quick.
Personally, if I were out for more than a few weeks in the middle of winter, I'd look at getting winter tires over chains.
(PS, one of my favourite storm-day activities, other than touring up into the forests for some private powder skiing, is to stand outside the Stone Bar in Argentiere with a mug of vin chaud, watching the carnage of people overestimating the abilities of their vehicles in bad weather... cars, coaches, trucks, everything. It's equal parts terrible and hilarious to watch a coachload of Spanish tourists slide backwards into and through a line of six parked cars, just because the driver thought he was good enough to not need chains. Don't be like him.)
Post edited at 12:12