Here is a photo essay documenting a recent window replacement at my house. A good friend of mine is a builder and came over to "give me a hand" replacing three windows. (Really, he did most of it and I provided some "semi-skilled" labour...)
This is a rather high-level look at the process. I will not be going into huge detail.
This is the "eating nook" in our kitchen. The house dates from 1984 and the kitchen windows are original, as far as I know. As you can see, the centre window is quite large, but it has one enormous flaw. The horizontal board that divides the two parts of the window is almost exactly at eye level when you are sitting at the table.
Outside view… My friend came by a few weeks ago and measured the windows. There are a few ways to measure and order windows. In this case, we measured the outside dimensions of the brickmold (the trim around the outside of the windows) and the manufacturer would make replacement windows to fit those dimensions, so that we would not have to make any changes to the outside siding of the house.
Our original plan was to save and re-use the interior trim. This plan had to change, since the new windows were not quite the exact same size as the others. (The outside dimension were the same, but not the inside.)
Using a pair of putty knives, I slowly worked the casing away from the wall. This is a slow process, to avoid damaging the walls. A lot of wiggling and gentle twisting produces the best results. Patience! Each time I might only pull the trim out 1/8-1/4", then move the knives and wedge in a different spot, then come back and work it looser.
The end results -- all interior casing is removed from the windows. It was interesting to note how tight the fit was on these old windows. There was barely 1/4" gap all the way around these windows.
I checked and verified that there were NO nails or screws through the window frames into the walls. This is quite common with new construction, I'm told. The windows came with the brickmold attached and were fastened to the studs with nails through the brickmold.
The first step in the removal process is to take your utility knife and cut through all the caulking that surrounds the windows.
Using a prybar, he worked on separating the brickmold from the window frame. He was careful here to always use twisting motions, more than prying. Always twist against the window frame itself -- watch out for the glass. Never pry from the outside, or we risk damaging the siding.
The first window is out, and the opening cleaned up. We checked and were pleased to find that there was no evidence at all of water incursion. For all that we didn't like them, the original windows did do a good job.