There are a ton of powerpoint templates for posters on the www, e.g. type
"poster template" into google.

Two that seem possibly nice to me are 

(a) http://www.writing.eng.vt.edu/posters.html (nice designs, wrong size)

(b) http://www.posterpresentations.com/CMS-Poster_template72x48.ppt (right
size, boring design)

Also

(c) http://www.stanford.edu/dept/VAS/posters/PPT_44inch.zip 

has some relevant templates (a bit boring) and some useful advice (though 
some of it is not so useful)