Suitable download for DVD decoder to replace lost...

Question

Computer H/D was reformatted and windows XP was reloaded by a friend (who has just gone on holiday). I'm assuming there must have been a disc for this but it seems to be missing. I've tried down loading a decoder from free ware but the same message still comes up "Windows Media Player cannot play this DVD because a suitable DVD decoder is not installed on your computer. A Sont DVD Rom player was installed after we purchased the computer.There must have been one on before as we did watch the odd DVD on it. Any help would be very gratefully appreciated.Thanks Jan