Problem With Context Switching Feature In SQL 2005

Hi,

One of our client application launches a execuatable on server by calling master..xp_cmdshell. We are migrating to SQL 2005 and I would like to use the Context Switching feature

We have sql login ��client�? which is used by clinet application and has very least privelge and have only required access to requried database and tables.
xp_cmdshell is disabled by default on our server and I would like to enable xp_cmdshell, launch the executable on the server and disable xp_cmdshell.I have created another SQL Login ��Bob�? and is member of sysadmin. The below SP is created by logging in as ��Bob�?. My objective is to grant execute access on this SP and grant impersonate access to sql login ��client�? . So that client application connecting to SQL server can launch the executable by calling the SP

create proc Bob.[Bob_enable_xp_cmdshell]
with execute as 'Bob'
as
EXEC sp_configure N'show advanced options', N'1' RECONFIGURE WITH OVERRIDE
EXEC sp_configure N'xp_cmdshell', N'1'
RECONFIGURE WITH OVERRIDE
EXEC sp_configure N'show advanced options', N'0' RECONFIGURE WITH OVERRIDE
-- launch the application using xp_cmdshell and disable the xp_command shell

When I try to execute (exec [Bob].[jtr_enable_xp_cmdshell]
) the above SP by logging in as ��Bob��, I��m getting the below error


Msg 15247, Level 16, State 1, Procedure sp_configure, Line 94
User does not have permission to perform this action.
Msg 5812, Level 14, State 1, Procedure jtr_enable_xp_cmdshell, Line 4
You do not have permission to run the RECONFIGURE statement.
Msg 15123, Level 16, State 1, Procedure sp_configure, Line 51
The configuration option 'xp_cmdshell' does not exist, or it may be an advanced option.
Msg 5812, Level 14, State 1, Procedure jtr_enable_xp_cmdshell, Line 6
You do not have permission to run the RECONFIGURE statement.
Msg 15247, Level 16, State 1, Procedure sp_configure, Line 94
User does not have permission to perform this action.
Msg 5812, Level 14, State 1, Procedure jtr_enable_xp_cmdshell, Line 7
You do not have permission to run the RECONFIGURE statement.


Please note the SQL Login ��Bob�? is a member of sysadmin fixed role. Thank in advance for you help.

With regards
GK