If its upvc frames its alot easier to replace, newer wooden frames are not too bad. You dont replace the whole window just the sealed unit. I have a little merlin laser tool which tells me the thickness of the glass so i dont have to take the glass out untill i have to replace it, and i just measure the width and height of the glazing beads (holds the glass in) take approx 7mm off each and order the glass from a local glass supplier. Or you can pop the glass out and measure the unit and thickness to be sure. Just need to make sure if its an opener or a door, you toe and heel the glass, ie using packers to keep the unit in place ect. If you go to your local glass supplier, they will be able to tell you all about it, or give you a leaflet, or just do a search on the net.