Often people call me after they've tried to remove the wallpaper and found it was too difficult.  In most cases, it is preferable to remove existing wallpaper before installing new or painting.  Sometimes it is necessary to prime over the paper with an oil based primer/sealer.  I recommend this only if removal would cause extreme damage to the drywall.  Whichever the case, I have the techniques to make your walls as good as new.  Sometimes the situation is so bad, customers have debated on replacing all the drywall, but even in the worst scenarios, working with the existing walls is less expensive and will still acheive the results you desire.