The following formula will return a text string of "10/31/05 - 11/06/05" if you enter the number 45 in cell A1. This is based on using serial date number 38348 (12/27/04) as the starting point for counting weeks.
=TEXT(38348+(A1-1)*7,"mm/dd/yy")&" - "&TEXT(38348+(A1-1)*7+4,"mm/dd/yy")