Vista - System.UnauthorizedAccessException with custom service

Hello

I made a service in vb.net that needs to read a file stored under c:\temp.
The service is well installed in the services configuration manager and run
with the local system account (by default). The service is retrieve the error
: System.UnauthorizeAccessException, Access to the path c:\temp is denied. I
setup th security on the file and on the directory but same error. What are
the security setting I have to setup to allow the service to read the file?

Here is the .net code used to read the file

Dim v_reader As New StreamReader(v_mailTemplatePathParameter.Value,
Encoding.Default, True)

Thanks for your help