I have a 3 month old Bichon whom i am housetraining.  I also have a 20 month old Collie cross who has started weeing against walls in the house since the puppy came even though he is (was) fully house trained.  Can anyone tell me why he has started doing this and how i can stop it?  Thanks

He is doing it to tell the puppy that he is the main dog and is also telling you that he is upset that there is a new pup in the house. How do they get on? If they are ok then it should pass once the pup starts going outside. Just keep cleaning it up, but don't make too much of a thing about it. If you catch him at it then you obviously stop him and take him outside. Just go back to basics with him.

my dog started doing this when we got a new pup maybe what the other post says is right but i dont know why they do it

The most important sense to a dog is that of smell, and marking territory is normal, and will occur more when there is an interloper on the dogs territory to make ownership of the territory clear.

In the wild these scent signposts are vital for animals to avoid needless confrontation.