Starting a workout as strenuous as p90x, you are guaranteed to be so sore it hurts to breathe.  This soreness will get better as your body adapts to your new workout regiment.  Many never make it to that point because they quit due to not being able to move without crying.  Here are some tips to help you make it through the soreness:

• Stretch
  If you are doing p90x, x-stretch is the BOMB.  You don't have to do the whole thing and you don't have to wait for it to be in the schedule.  Do it when you need it.  Also, be sure you are stretching properly before and after each workout.
• Protein Shakes
  Drink a protein shake after you workout.  It will help your muscles rebuild and heal faster.
• Hydrate
  Make sure you are drinking at least 8 glasses of water a day.  
• Swim
  The cold water helps reduce inflammation, and moving around in the water helps stretch out your muscles and ease soreness. 
• Massage
  A gentle massage can help ease the soreness and stiffness.
• Sleep
  Make sure you are getting plenty of sleep so your body can recover adequately.