How to get the current user's Active Directory details in C#

I am working on an C# and ASP.Net application, that uses Windows Authentication.

i.e. in Web.config:

<system.web>
    <authentication mode="Windows" />
</system.web>

I want to get details for the current user (full name, email address, etc) from Active Directory.

I can get their pre Windows 2000 user login name (eg: SOMEDOMAIN\someuser) by using

string username = HttpContext.Current.Request.ServerVariables["AUTH_USER"];

I've worked out the LDAP query for the user, using their current login name (not their pre Windows 2000 user login name):

DirectorySearcher adSearch = new DirectorySearcher(
        "(userprincipalname=someuser@somedomain.com.au)");
SearchResult adSearchResult = adSearch.FindOne();

However, I don't know how to either search AD for the user using their pre W2K login name, or get their login name in the 'someuser@somedomain.com.au' format.

Any ideas?