My 5 month old miniature schnauzer is constantly barking at anyone who dares come into our house!!  I am happy if she barks at the bell or at noise of someone coming up to drive but she will bark constantly if they come into the house!  She will settle for a little while and then starts again if they get up or leave the room.  What can i do to try and teach her that visitors are nice people and not 'the enemy'!!!

Your puppy is trying to communicate that there is someone there so acknowledge the fact " thank you- that will do" and then ask for another behaviour like down sit or watch me and reward that, treat lots of fuss. Don't shout at the pup because, after all that is what he's doing - shouting at the people, you're just joining in.
When my dogs bark to tell me something (be it door or wants out) I always ackng. then praise for something I want... in my case with my adults, it's silence. You always need to remember to praise for the abscence of a behavior you dont want..... eg praise for being settled or for being quiet or not jumping up. Don't always wait till pup does something wrong and then reprimand.  IMO it's better to be a positive trainer/owner not a corrective one. We all learn by success dont we, a pat on the back is always better than being nagged at.