paint removal not needed unless you want to stain, scrape worst off then sand well, if using stripper still got to sand anyway so no need. as for your windows there is not a lot you can do because the sun causes it to go yellow and any no matter which cleaning materials used speed up the discolouration process (fact!) the only way to care for upvc is to wash reguarlly with a very mild non abrasive cleaning solution. you can use acetone pvc cleaner or you can get pvc cream cleaners but both in the long run make it worse sorry for the bad news but in my eyes upvc windows/doors are the worst invention ever i make and fit bespoke timer frame windows and doors and they last a lot longer than upvc if looked after