Dynamically set impersonated username and password

Hi,
I have an ASP.Net application that is working well with a Windows authentication mode and impersonation.
An example of the Web.config file is currently as follows:
<authentication mode="Windows" /> <identity impersonate="true" userName="mydomain\username" password="th1s1s1t000" /> <authorization> Now there is a requirement to store the username and password values in an encrypted form in the Web.config file. Then use methods within the code to decrypt the values.
So I would like to set the impersonation values(username and password) from within the code. I am not sure at which point of the lifecycle of the web page I can set this, and the code that I can use.
Could anybody help?