INSERT WHERE NOT EXISTS

Hi all,

I've have a need with SQL Server 2005 (so I've no MERGE statement), I have to merge 2 tables, the target table has 10 fields, the first 4 are the clustered index and primary key, the source table has the same fields and index.

Since I can't use the MERGE statement (I'm in SQL 2005) I have to make a double step operation, and INSERT and an UPDATE, I can't figure how to design the WHERE condition for the insert statement.

Any help?