If you have MS Office, it may have included Publisher. I just checked (never had a use for it) to see that it does contain templates for banners, some of which may be suitable for posters. It may also handle splitting the image into sheets for you, though I haven't checked that. When I do banners, I use the HP DesignJet printer at work; you'd be surprised at what you can make Windows Picture and Fax Viewer do. My record is a 22' x 24" banner of the Colorado River, printed from a panorama of 14 separate photos. :)
"A Journey of a Thousand Rest Stops Begins with a Single Movement"