How to repair or replace rotted wood below the frame of large windows?
There's an alternative to replacing the wood; there are several ways to repair it using resins.


One approach involves four steps.  First, any loose material is removed, but intact, soft material can remain.  Second, a thin resin is applied to penetrate deeply into the wood, seal it, and provide a solid surface.  Third, an epoxy putty, often filled with wood fibers, is used to fill and restore the original shape.  The last step is sanding and finishing.  This link illustrates the process: http://www.hereandthere.org/oldhouse/fixing-decayed-wood.html.
A simpler three-step process is to grind away the rotted wood down to good, solid wood.  The void is then filled with a special epoxy putty that strongly bonds with wood, remains a little flexible, and expands and contracts similar to wood.  The putty is shaped and contoured to match the original.  If the original isn't a simple flat surface, the shape can be reproduced on a plastic scraper that is then used as a template to shape the putty.  When it hardens, it is sanded and finished.

Here's a video of this type of repair: https://www.youtube.com/watch?v=3l5q0xaQEf8.  As noted in the comments, the video uses an extremely expensive material.  But you can accomplish the same result with much cheaper materials.  For example, Bondo and others make special epoxy putties designed for rotted wood repair.  Here's a link to the Bondo product: http://bondo.com/bondo-rotted-wood-repair-kit-20178.html, and a link to one sold by Home Depot: http://www.homedepot.com/p/PC-Products-Rotted-Wood-Repair-Kit-084113/100664090.