You're better off running PowerDVD especially if you have the latest full version running there. That will also run audio cds along with a choice of screen savers it comes with. Sound familiar? The only WMP 10 is used for here is the abilty to create your own playlists with mp3s.
It seems WMP no matter which version of it you get is "always" missing one codec or another. If I am not running a playlist I may throw in a video cd at times to see how well it came out and if it can be run there. But that's about it for WMP!
WMP doesn't enclude a DVD Decoder, so you need to have a third party DVD program or codec. The demo of Nero also includes one if you want one for free.
All you need is a codec, and WMP10 will play DVDs just fine. You don't need another program. Download and install the following codec pack. You'll never need another codec again.
Are you seeing a blank listing for drives when going to the Play drop down menu and choosing the "DVD, VCD, and CD AUDIO" link? That can be caused by the format of the media you are playing at times. I often run into that when trying to run a dvd-r. The codec there although updated was originally written for WMP 6. Keep PowerDVD regardless. It's an excellent player.