Shell not executing

[Guest]

my asp.net application is running under an impersonated account.
I use office automation to call Excel from this application.

since I wasn't able in no way to kill the excel process after using it, I
used the pskill
utility from "sysinternals.com" to kill it, by trying to use :

Shell("c:\pskill Excel")
this code run without any error thrown but the excel process is not killed
and it still exist in task manager window

OR

Dim p As New System.Diagnostics.Process
Dim pi As New System.Diagnostics.ProcessStartInfo
pi.FileName = "C:\pskill.exe"
pi.Arguments = "Excel"
p.StartInfo = pi
p.Start()
the same with this code too: no error but the excel process is not killed


is there anything wrong with both code or it's because of the impersonation
that the application using, and if so, what permission I can assign to this
account in order to kill a process and how ???

thanks for help