ASP.Net which user account running Web Service on IIS 7?

I want to know which account running my Web Service/Application so that I can assign the read/write access to that account. I have researched and see most of the sources mentions about ASPNET account, but on my 2008 server, there is not any acount named ASPNET. Some sources say ASPNET is being replaced by NETWORK SERVICE?

Also, can I assign to another user account to run the service? And how to do that?

Thanks,