What are the minimum requirements to commute comfortably through the winter? What needs to change on my bike? I have an 80's Schwinn Traveler single speed conversion.

What needs to change with my clothing? I just wear regular street clothes now.

I'm looking for the minimum requirements to commute comfortably through the winter. I am open to modifying my summer bike but not if it is likely to get ruined. I am open to buying a "beater" specifically for winter. I can bring my bike inside at home, but not at work. I'm in NYC.
The bike doesn't matter that much, except when there's snow or ice on the roads.  What you need most is the right clothing.  It needs to be well-adapted to cycling (not loose or apt to get caught in the chain), layered so you can take off pieces BEFORE you get too warm, with wind-resistant and rain-resistant layers.

Your shoes need to be somehow protected from wet spray, with rubber booties or such.  Wet feet can get VERY cold.

Lots of opinions on gloves -- you should probably have several pair, from warm "ski gloves" for really cold days to lighter-weight gloves for most days.  "Glove liners" are a good idea -- you can use them alone (under regular cycling gloves) on warmer days, and they come in handy when you need to work on the bike in cold weather since you can work reasonably well with them on.

For the bike, any bike will work for plain rain or cold (though fenders are nice in the rain).  For riding in actual snow (where the roads have not been cleared) you want a substantial tread, and possibly lugged "mountain" tires.  For any sort of ice you MUST have studded tires -- riding on ice without studded tires is suicide.

I don't feel that commuting is apt to "ruin" a bike, but you probably wouldn't want to subject a $6000 bike to the bad weather, road salt, being parked outside at the other end, etc.  And if you do want to ride in the snow and don't already have a bike suited to knobby tires, it does make sense to purchase a modest "mountain" bike.

Added:  Forgot to mention the head.  I always wear a helmet, making a bulky hat impractical.  At one time I had a "helmet liner" (probably purchased from Performance or Nashbar) that was a sort of diamond-shaped piece of blue polypro fabric with loops to fit the helmet straps.  This worked well -- kept the ears warm in fairly cold weather, which is the main challenge -- but I lost it and never found a replacement.

I've tried several balaclavas and face masks but never found one that would fit comfortably, without either obscuring vision or interfering with breathing.