This video below is a great aid to help you install new PVC windows. The guy in the video goes through the whole process of installing them and making a perfect job to boot. He takes you through how to measure the windows beforehand so you have the exact right size and how to take the packers around the side of the new windows out so it is ready to install.
He also shows you how to cut through the old frame without causing any damage to the surrounding area and then he shows you how to use wedges and then fit in the new PVC window and secure it. What is now required is to pump foam into the cavities to fill them up so the windows will be well insulated and keep the warmth inside the house. You then need to wait awhile until the foam has fully set.
From there he shows you how to use spacers to ensure the whole window has a sound and tight structure. He then uses a block of wood and a rubber mallet to knock the slits back in. After this it is a matter of tidying up and doing any filling in, maybe putting some silicone in around the outside, and then you are done!!
The guy in the video makes it all look rather simple and straightforward but there is no doubt that it does take a lot of skill and expertise to do this correctly and make sure you have done a perfect job so it is the right fit and fully insulated. I would say that if you are at all unsure about tackling this type of job by yourself leave it alone and get a professional in to do the job for you. It may be that you are very good with your hands and feel comfortable tackling such a project, in which case go right ahead.
Here is the video for to check out:
For installing windows and doors, if you do not want to do some DIY work yourself then one suggestion would be to find a reputable company to do this for you such as Smyth Window Systems in Portadown, Northern Ireland. They have been installing PVC windows and doors for over 15 years and have an enviable reputation as a first class company. The main thing before you use anyone is to check them out as thoroughly as possible so you know you have made a good choice.