Drive it.. Drive it..
Split the middle.. Don't drive it with snow/salt on the ground but do use it a few times a month. Its a nice escape. Heck, down here near DC last winter there were at least 10-15 days of top down driveable weather (ie above 60 degrees) between November and March and another 10-15 of "top up" but still nice driveable weather as well.. Thats 30 days right there! A '99 Boxster won't depriciate much more anyway, so you may as well enjoy it.
