See if this helps.
Making a DVD Video using Nero 7 Premium

If your WMP is missing codecs, you'll have to install codecs before you'll be able to watch movies with WMP! Here are links to codec packs, the XP Codec Pack & K-Lite are popular.
CODECs list for Movies/Alternate players / Video help forums/ video-mp3 fixers

It's up to you which media player(s) you keep or ditch. Have a look at them all & work out which one(s) you like best...which ones have the best features, which are user friendly

To find out what version of WMP you have, click on help > about WMP & you'll see.