Problem At Constarints

hi,
here i am trying to create a table in which i want give a property autonumber(i.e increments simultaneously for each new row) to a field.pls tell me the procedure for this in sql server2005


doubt2:
i had created a table before with one primary key column. next i want to add a new column to the table and it also be a primary key.
what is the procedure to add a primary key constraint column in a table if that table has already a primary key. i want multiple columns must contain primary key consatrints.

thanx in advance,
rammohan