Juan (and Kuk) --
I went to Europe this past January, although for only 3 weeks. Saw England, Brussels, Netherlands, Germany, France, and Ireland -- all pretty briefly, obviously. The only *really* cold place I went to was Berlin -- everywhere else I went was pretty moderate (although Ireland rained every day, of course, which made it seem much colder, being damp all the time).
I have to disagree with some people about France. I went there expecting the worst, and was very pleasantly surprised. Paris was the most amazing city I saw, and the people were very friendly, overall. I think was for two reasons, mainly: (1) I made attempts (however pathetic) to speak basic French when I interacted with people, rather than assuming they knew basic English, and (2) there were not many tourists around in January. I have read that in the summer months Paris is so overwhelmed with tourists, that it's like they run the city. I could understand how anyone might be upset by this -- but obviously, it has contributed to Americans' view of the French, and vice versa.
Basically, just don't act like an ignorant, arrogant American, and you will be treated respectfully in return, for the most part.
Although some more 'serious' backpackers would deride it, I highly suggest the "Let's Go Europe" book -- it has great, basic information about all of the countries, and hostel listings as well. The Eurorail offices in any major city can help you plan your travels and find the most appropriate train ticket.
So anyways, pack some warm clothes, but I wouldn't worry about the cold too much, unless you're going way north. Besides, the weight of the backpack, and walking around all day will keep you somewhat warm. I remember sweating profusely in the snow one day in Berlin.
Good luck, enjoy, and email me if you have any further questions that I might be able to answer!