If you want to impersonate a WindowsIdentity in ASP.NET you can use the Win32 LogonUser API and construct a WindowsIdentity obeject from the token and attach the token to the current thread:

// Declare signatures for Win32 LogonUser and CloseHandle APIs
    [DllImport("advapi32.dll", SetLastError = true)]
    static extern bool LogonUser(
      string principal,
      string authority,
      string password,
      LogonSessionType logonType,
      LogonProvider logonProvider,
      out IntPtr token);
    [DllImport("kernel32.dll", SetLastError = true)]
    static extern bool CloseHandle(IntPtr handle);
    enum LogonSessionType : uint
    {
        Interactive = 2,
        Network,
        Batch,
        Service,
        NetworkCleartext = 8,
        NewCredentials
    }
    enum LogonProvider : uint
    {
        Default = 0, // default for platform (use this!)
        WinNT35,     // sends smoke signals to authority
        WinNT40,     // uses NTLM
        WinNT50      // negotiates Kerb or NTLM
    }

protected void Impersonate(object sender, EventArgs e)
    {
        IntPtr token = IntPtr.Zero;
        WindowsImpersonationContext impersonatedUser = null;

        try
        {
            // Create a token
            bool result = LogonUser("User", "Domain",
                                    "Password",
                                    LogonSessionType.Network,
                                    LogonProvider.Default,
                                    out token);
            if (result)
            {
                WindowsIdentity id = new WindowsIdentity(token);
                // Begin impersonation
                impersonatedUser = id.Impersonate();
                // Resource access here uses the impersonated identity
            }
        }
        finally
        {
            // Stop impersonation and revert to the process identity
            if (impersonatedUser != null)
                impersonatedUser.Undo();
            // Free the token
            if (token != IntPtr.Zero)
                CloseHandle(token);
        }
        // Verify the old process identity
    }