To get the table's primary key you can also use:

sp_pkeys tableName

if table is is different schema then dbo

use following

sp_pkeys  TableName, SchemaName

One another way of finding all PK in a DB

SELECT 
    SO.[Name] as [Table_Name],
    ColUsage.Column_Name as [Column_Name],
    TC.CONSTRAINT_NAME as [Primary_Key]
FROM 
    INFORMATION_SCHEMA.TABLE_CONSTRAINTS TC, 
    INFORMATION_SCHEMA.CONSTRAINT_COLUMN_USAGE ColUsage ,
    (SELECT [NAME] FROM dbo.sysobjects WHERE xtype='u') AS SO
WHERE 
    ColUsage.Constraint_Name = TC.Constraint_Name
    AND ColUsage.Table_Name = TC.Table_Name
    AND Constraint_Type = 'PRIMARY KEY '
    AND ColUsage.Table_Name = SO.[Name];

Another way of finding all PKs in a DB:

SELECT 
    TC.TABLE_NAME as [Table_name], 
    ColUsage.Column_Name,
    TC.CONSTRAINT_NAME as [Primary_Key]
FROM 
    INFORMATION_SCHEMA.TABLE_CONSTRAINTS TC
    INNER JOIN 
    INFORMATION_SCHEMA.CONSTRAINT_COLUMN_USAGE ColUsage
    	ON TC.CONSTRAINT_NAME = ColUsage.CONSTRAINT_NAME
WHERE 
    TC.CONSTRAINT_TYPE = 'PRIMARY KEY' 
    AND TC.TABLE_NAME 
    IN (
    		SELECT [NAME] AS [TABLE_NAME] 
    		FROM SYS.OBJECTS 
    		WHERE TYPE = 'U'
    	);