ubns,
I'm thinking that something like the below is what you are wanting.
Right click your sheet tab >> View Code >> paste the following into the code pane.
Code:
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Cells.Count > 1 Then Exit Sub
If Target.Row < 5 Or Target.Row > 1000 Then Exit Sub
If Target.EntireRow.Cells(1, 1) <> "" Then Exit Sub
nr = Cells(Rows.Count, 1).End(xlUp).Row + 1
MsgBox "Please enter an ID in the next available row before attempting to add other data"
Cells(nr, 1).Select
Application.EnableEvents = False
Target.ClearContents
Application.EnableEvents = True
End Sub
Hope that helps.