I have read the first third of this thread, but I still wanted specific advice on my questions.
I want to commute to university throughout this school year, which, being in Michigan will constitute a fair amount of wet/snowy weather. The commute is an easy 2.5 miles, mostly on a road with a bike lane, and then small stretches of parking lot and bike paths.

My current bike is a 2005/6 Specialized Rockhopper comp, with tires that are knobby on the outside and slick on the inside, and front fork suspension. I like the way the bike rides for the commute.

My questions:
1) Once I get a good lock for the bike, should I worry about this kind of bike being stolen on a college campus? Maybe I should get a different bike?
2) I am going to get full fenders, but how do I know if the fenders will fit the bike/tires? I read elsewhere that if the tires are wider than 2" then they will rub on 60mm fenders. When I tried to measure my tire width, they appeared to be more than 2" wide, but maybe I'm not measuring from the right place.
3) Would I need to get different tires for winter?
4) All of the padding in my current helmet is disintegrating, including the part that attaches from the back of the helmet to the chin strap. Can I replace this, or do I need a new helmet?

Thanks!