Unfortunately, the System.Windows.Forms is not supported in Windows Store Application.
You can see all .NET assembly available on Windows Store App on this link :
http://msdn.microsoft.com/en-us/library/windows/apps/br230232.aspx
Your application may not run on the Windows RT tablet, but tablets running full Windows 8 will be fine (Microsoft Surface Pro).
Do simple Windows Forms/WPF apps work on Windows 8 for tablets?