Enabling/Disabling constraints

-- Enable Constraints
ALTER TABLE tablename CHECK CONSTRAINT constraintName
ALTER TABLE tablename CHECK CONSTRAINT ALL 

-- Disable Constraints
ALTER TABLE tablename NOCHECK CONSTRAINT constraintName
ALTER TABLE tablename NOCHECK CONSTRAINT ALL

-- Add constraint, without checking existing data -
ALTER TABLE tablename WITH NOCHECK ADD CONSTRAINT PK_tablename_id PRIMARY KEY(id) 

Solace : Disabling All Constraints (use with caution, e.g for data loading)
Ode to Code : SQl Server Constraints
Joe Webb : Disable constraints on a table