Download and install My Movies 3 for Windows Media Center
Download and install DvD Decrypter
Insert the Dvd you want to put in WMC, select Main movie + IFO file for decryption
Select a destination folder, when decrypter is done you will want to go into that folder and rename the movie you just put on your hard drive.
Now open WMC and go to Settings/media library/movies than select the fold you want WMC to search for movis.
This is what I have wanted to do for years. My Movies 3 is amazing. Every time I add a movie it updates the file in WMC with the cover art and all the other info about the movie (director, actors, IBDM rating, etc.)
And with my wife's new laptop on my homegroup we can watch movies that are on my HD from her laptop, through WMC, with access to all the My Movies 3 options as well.