you need a specific paint that adheres to Upvc, and the windows will need to be sanded down with a fine grade wet and dry sandpaper. There are companies out there that will paint upvc windows for you, my concern would be overspray on rubber gaskets, glass , hinges etc. If you are after a ' grained ' wood looking effect like Medium Oak Upvc windows then that is achieved by a ' foil ' when the profile is manufactured and won't ever be achieved by painting them