I'm not sure which buses won't be running...do you mean the busabout kind? Certainly you can travel on Eurolines (www.eurolines.com
) all year round, they mostly have point-to-point long distance trips and are quite reasonable.
Have you checked into rail passes, as they can make the train fares a lot more reasonable in western Europe (click on the RailEurope banner for details)?
A lot will depend on where you are travelling, but people still move around a lot in the winter over here, so there will be options. Tell us where you're going, and we'll help you get there!