Short of a studded winter tire, I'll throw in the option of a cyclocross tire with "mud" style treads. After having used both, I've found that non-studded knobby tires are much more efficient and comfortable to ride. I cant stand the rolling resistance of studs.
However, most road bikes wont fit tires wider than 28 mm, so you might not have many options here. Usually the limiting factor is either the narrow style calliper brakes that most road bikes use, or the narrow clearance at the chainstays.
Your 25 mm tires will do okay in slush or light snow up to 1 inch deep, but forget it on anything deeper.
Time to by a beater mountain bike for winter! Every cyclist should have a $25 Craigslist Mtn Bike for winter commutes
