I've got 2 road outfitted bikes that work well with 32mm tires in anything up to slush and light snow dustings. I can throw some file-tread CX tires on there and it's all good.

For real deal winter: Packed snow, ice, etc. I'll throw a pair of studs on my 29er/CX rig, smack a couple of clippie fenders on it and be ready to rock the gnarly stuff.