The xp_cmdshell option is a server configuration option that enables system administrators to control whether the xp_cmdshell extended stored procedure can be executed on a system.

-- To allow advanced options to be changed.
EXEC sp_configure 'show advanced options', 1
GO
--To update the currently configured value for advanced options.
RECONFIGURE
GO
-- To enable the feature.
EXEC sp_configure 'xp_cmdshell', 1
GO
-- To update the currently configured value for this feature.
RECONFIGURE
GO

References
http://technet.microsoft.com/en-us/library/ms190693.aspx