Installing uPVCs windows are rarely done by DIY homeowners. It is a complicated procedure that is much harder than installing a sub-floor or tiling for example. Of course any uPVC dealer will offer installation in the price of any deal you might sign.
Before committing to any firm to install your uPVC windows it is a good idea to do a bit of research and get a few quotes. It could be cheaper to try for a discount on the uPVC windows and find a cheaper local contractor to do the installation. It never does any harm to ask about testimonials and ask about any relevant references.
Having said all of this, most installations go smoothly and can be done in a few hours. Here is a great video showing the installation of uPVC windows: