Preventing multiple forms from loading?

On my mnuAbout click event I have the following.

Dim fAbout as New frmAbout
fAbout.Show

if the user keeps clicking mnuabout it will keep loading new forms. How do I prevent more than just one?

VB Code:

Private fAbout As frmAbout 'Declare at form level
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    If fAbout Is Nothing Then
        fAbout = New frmAbout
        fAbout.Show()
    End If
End Sub

Hmmm that sorta works but now how to make fabout = nothing when the form is closed?