Polyvinyl chloride, or PVC, was invented by Dr. Waldo Semon, an employee of the BF Goodrich company, in 1925. He was trying to make a material that would bond metal and rubber; his accidental discovery of PVC was all but unused for ten years following. In the 1930s, PVC was turned into tire treads, and two decades later into PVC pipe.
PVC is used for its strength, shock absorption and durability. Especially in earthquake-prone areas, you may find PVD windows and doors as well as pipes. If for some reason your PVC window has been painted, it's important to be careful in removing the paint to avoid damaging the window.
If the PVC window in question has been painted with a matt emulsion paint, warm water should go a long way in softening it for removal. Soak a sponge in warm water and wet the paint. If it softens, you may be able to simply wipe it off; otherwise, gently coax it off with the scraper.
If your window was painted with gloss paint, it also shouldn't be too hard to remove. Gloss paint doesn't adhere very well to plastic. Try using the scraper alone in this case and see if a few firm, but not too vigorous, strokes can get some of the paint off. Try first in a small corner of the window to make sure you don't scratch the PVC.
If neither of these steps works, try some everyday paint stripper. Depending on the make-up of the paint thinner you use, it may damage your PVC, so again, try it in a very small area in a corner of the window to make sure it won't damage your PVC before applying it more widely.