With replaing old existing windows is there any effect on the outside of the home or is it just removing the old trim and popping out the window and reaplcing it then new trim?

A lot depends on the condition of the current window condition. How the casings look and the sill. Also what types are you working with? Was the old windows the double hung style and now your going with aluminum frame style? What is on the exterior of the home? Sometimes all the banging can caus cracks to appear in stucco homes.

Are they custom windows, or off the shelf, is there problems with the wood behind the windows

If u are doing a total window replacement, the trim and sill on inside have to be removed. The new windows will come with exterior trim on them. U may have to add or remove some siding to get it in. The top of window needs a drip cap that goes under siding. Framing should be wrapped with 15 lb felt from bottom to top.A layer of 15 lb felt should be set from window edge toward siding. Sidle under siding to use as a water shed. Install window and finish siding to it. Insulate and trim inside.

If installing replacement windows, u have to remove the sashes and sashes wheels/ballasts parting strip and inside stop. Put caulking on outside stop and install window against it. Reinstall inside stop and u are finished. The outside trim on existing window frame does not have to be touched.

Hope this helps.

Chuck

Agreed, but you can order replacement windows that reuse parts of the existing window frames and sash. This should be a good read Windows of Opportunity: DIY Window Replacement | Extreme How To, DIY - Do it Yourself, Home Improvement, Home Decorating