You really shouldn't do that - it is the default and expected behaviour for windows, and you mess with that at your peril. Users do know that ENTER activates a button, and if you don't specify a button to "accept" the ENTER, the currently focused control gets it.
The best solution would be to make ENTER press the "Equals" button - which is simple to do.
In your form, override ProcessCmdKey:
Protected Overrides Function ProcessCmdKey(ByRef msg As Message, ByVal keyData As Keys) As Boolean
If keyData = Keys.Enter Then
buttonEquals.PerformClick()
Return True
End If
Return MyBase.ProcessCmdKey(msg, keyData)
End Function