Exec Stored Procedures with high level user permissions

I need to run a stored procedure with admin rights inside the stored procedure like so

Exec xp_cmdshell "c:\start.exe"

i need this to have administrator access so when it is called upon by an asp.net web page that executes the stored procedure it is not run by Network Service user but Administrator. How would i do this.it's the account that runs sqlserver serice that you need|||i am sorry i still don't understand if you could give an example stored procedure or

are you stating that the user that stored the stored procedure is the user level it is run at.|||ohh i forgot to add thank you for replying so fast last time|||what i am saying is that you need to make sure that the account that is used to start sqlserver service is an account that is given enough priveleges to perform the task you need.|||so if the service is started by user Administrator which it alwasy is. That is the user used to start any programs in the stored procedure.