Good afternoon to ol!

I have an MDI form which, naturally, opens several child forms. My question is how do I prevent the user from opening a form that is already open? To load the child forms I am using the following syntaxes:

dim frm as New PurchaseOrder
frm.Show()

I did not make use of the ShowDialog method because I want the user to be able to load other forms as well. If the user tries to open a form that is already open the system should just bring that form on top of all other forms.

Any ideas? Thanx! in advance!!!