Use this:=

IF NOT EXISTS (SELECT value_in_use

FROM sys.configurations

WHERE name = 'xp_cmdshell'

AND value_in_use = 1)

BEGIN

EXEC ('sp_configure ''show advanced options'', 1')

RECONFIGURE;

EXEC ('sp_configure ''xp_cmdshell'', 1')

RECONFIGURE;

EXEC ('sp_configure ''show advanced options'', 0')

RECONFIGURE;

END