[font="Verdana"]You cannot modify system tables in Sql Server 2005 normally but you can modify the same if you are connected using Dedicated Administrator Connection (DAC). Refer the below link regarding the same,

http://msdn2.microsoft.com/en-us/library/ms189595.aspx

Please note that it is NOT advisable to modify system tables and if you perform the same it is at your own risk.[/font]