[goggles99]

hmm, for vb6 it can be done very simply using "App.PrevInstance".

Private Form_Load( )
     If App.PrevInstance Then
          Unload Me
     End If
End Sub

for vb.net it's a bit different, but this function should work

Function PrevInstance() As Boolean
If Ubound(Diagnostics.Process.GetProcessesByName(Diagnostics.Process.GetCurrentProcess).ProcessName)) > 0 Then
     Return True 
Else 
     Return False  
End If
End Function

There is also a similar way to do it in vb.net here
http://www.thescarms.com/dotNet/SingleInstance.asp