Forms Authentication Not Able To Save Programmatically

im using visual studio 2008. in my project im using forms authentication, my project structure is as follows

root
|
login.aspx
home.aspx
web.config
admin (folder)
|
admin.aspx
web.config

here in root web.config is as follows

<authentication mode="Forms">
<forms name="Authen" protection="All" timeout="60" loginUrl="Login.aspx" enableCrossAppRedirects="true" cookieless="AutoDetect"/>
</authentication>
<authorization>
<deny users="?"/>
</authorization>

and in inner web.config and my users are

Username Role
admin1 Admin
admin2 Admin
user1 User
user2 User

as you can see that in second web.config, i gave access to users with "Admin" role and "user1" user. for giving access to "admin" folder, i wrote the following:

Configuration config = WebConfigurationManager.OpenWebConfiguration("~/HR");
SystemWebSectionGroup systemWeb = (SystemWebSectionGroup)config.GetSectionGroup("system.web");
AuthorizationSection section = (AuthorizationSection)systemWeb.Sections["authorization"];
AuthorizationRule newRule = new AuthorizationRule(AuthorizationRuleAction.Allow);
newRule.Users.Add("user1");
section.Rules.Add(newRule);

and saved the web.config as

config.Save();

this is working fine when i run it from visual studio development server, when i host it in IIS, im getting the following error

Configuration Error
Description: An error occurred during the processing of a configuration file required to service this request. Please review the specific error details below and modify your configuration file appropriately.

Parser Error Message: An error occurred loading a configuration file: Access to the path 'F:dotnetsamplesmyprojUIhrf-q_g-yn.tmp' is denied.

Source Error:
[No relevant source lines]

Source File: F:dotnetsamplesmyprojUIhrweb.config Line: 0

here the problem is, it is not able to save the web.configi got one solution, that is, i added

<identity impersonate="true" userName="myusername" password="mypassword"/>

to root web.config and it is working fine. but my boss says that is not the correct way. what is the correct way to solve this problem