"Windows Media Player cannot play this DVD because a compatible DVD decoder is not installed on your computer."
OK this cause by missing decoder or the decoder not installed. If you browse Microsof website the solution is:
click start - run - type dvdupgrd /upgrade
OK, for me there is another solution that i like and my favourite which is installing codec packs. The price is FREE.
I recommend installing these codec pack to solve the problems:
K-lite codec pack (FULL)
XP codec pack
Not necessarily you need all of those, in my PC I install Klite Codec pack, tiger player, Real Player, Real alternative and Windows Media Player Classic.