Need a stored procedure to check and update row when it does not already exists.
without any more information it is difficult to give you the "right :)" answear. but what i can tell you, check:

http://www.techrepublic.com/article/defining-sql-server-constraints-with-tsql/6181836

and ...

When you specify a PRIMARY KEY constraint for a table, the SQL Server Database Engine enforces data uniqueness by creating a unique index for the primary key columns. If a PRIMARY KEY constraint is defined on more than one column, values may be duplicated within one column, but each combination of values from all the columns in the PRIMARY KEY constraint definition must be unique.

See more on http://msdn2.microsoft.com/en-us/library/ms175132.aspx here