$ie = new-object -com "InternetExplorer.Application"
$ie.Visible = $true

$win = Add-Type -Name win -MemberDefinition @'
[DllImport("user32.dll")]
[return: MarshalAs(UnmanagedType.Bool)]
public static extern bool SetForegroundWindow(IntPtr hWnd);
'@ -Passthru

$win::SetForegroundWindow($ie.HWND)