How to avoid a database race condition when manually incrementing PK of new row

Question

I have a legacy data table in SQL Server 2005 that has a PK with no identity/autoincrement and no power to implement one.

As a result, I am forced to create new rec

Answer 1

Create an auxiliary table with an identity column. In a transaction insert into the aux table, retrieve the value and use it to insert in your legacy table. At this point you can even delete the row inserted in the aux table, the point is just to use it as a source of incremented values.