White Christmas
It's a Wonderful Life
The Shop Around the Corner
Meet Me in St Louis (not strictly a Christmas film, but has a lovely Christmas scene)
Muppet's Christmas Carol
Scrooge (Alastair Sim)
Miracle on 34th Street (the original, find the newer version too saccharine)
The Bishop's Wife
Also have a soft-spot for those awful, channel 5 made for TV movies
PP, what about the Raymond Briggs cartoons? Father Christmas, The Snowman, The Snowman and the Snowdog? They're all quite short, my son adores Father Christmas, we watched it loads last year.