Windows Media Player XP

We (wife & I) 2 laptops (Toshiba) both OK to play DVD’s. 2 Desktops have Microsoft Windows Media Players installed (XP) but cannot play DVD’s. PC does not even recognise presence of DVD in drives. Both are WMP version 11. Mine has 3 decoders listed (bcVideoDecoder.dll)(CinemasterVideo)(, decoder checker indicates none are compatible. I also have VLC media player that will not pick up the DVD. I can play CD’s with VLC player but not WMP.
My wife has decoders, ( only last is regarded as compatible. She can play CD’s. PC picks up presence of disc immediately and provides standard option to select play CD.
Any ideas out there on how to correct this and be able to play DVD’s?