Can no longer play commercial DVDs on my Microsoft...

Question

Microsoft XP. Used to play commercial DVDs, eg movies on Microsoft Media Player. Doesn't now. Instead I get a message that I need a compatible DVD decoderMy sister has XP & can download CD music tracks for Movie Maker. When I try with the same CDs that she uses, I get a track protected message that prevents this. .Happens with all my CDsPlease advise. Many thanks. G Jones