SELECT 'EXEC sp_rename ''[' + c.CONSTRAINT_SCHEMA + '].[' + c.CONSTRAINT_NAME + ']'', ' +
   '''PK_' + c.TABLE_NAME + ''', ''OBJECT'' ' 
FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS c
WHERE c.CONSTRAINT_TYPE = 'PRIMARY KEY'
   AND c.TABLE_NAME IS NOT NULL
ORDER BY c.TABLE_NAME