WMP, of course, cannot play DVDs without the codec. Your ASUS DVD player may be an older version of WinDVD which is not compatible with WMP10. The reason WMP is not a stand alone DVD player is that the codec needed requires a licensing fee. MS is not going to pay that to run a free media player and users must do so. I would suggest uninstalling and reinstalling your ASUS player. This might be a case similar to losing plugins when updating a browser and needing to reinstall them. If this does not fix it, my guess is the codec is not going to work with WMP any longer.
I am running Win Xp Home. recently I acquired a broadband connection and updated everything. So now I have SP2 and WM10.
Earlier my Windows Media Player used to play my DVDs now it says that I need to download(buy) a DVD decoder for it to play DVDs!
Its not that much of a problem I already have the Asus player that came along with my GeForce but I would like to have Media Player also play DVDs