My 5 month month old lab, is great, trouble is when she sees another dog on a walk she stands their barking!!! But she will then go up to the dog and be as nice as pie!

How do i stop her doing this!

ASk her to do something else - such as 'watch me' - this can be trained by holding s tasty treat under her nose, then slowly withdrawing it as you stand up to chest level.  Then reward her with the treat.  Eventually you'll be able to hold a hand in that position and she will take it as a hand signal to watch you.