Making a macro run automatically

One way you can do this is to use the Worksheet_Change event. Try something like this:-

On your worksheet, right click the tab and click View Code. From the left hand combo box pick Worksheet and from the right hand combo box choose Change. This will create an empty procedure that will be carried out every time a change is made to any cell. For example, say you wanted to check that any cell value in column B to make sure that it was lower than the cell in column A (same row) you'd use something like this:-

Private Sub Worksheet_Change(ByVal Target As Excel.Range)
If Target.Column <> 2 Then Exit Sub
If Target.Value > Target.Offset(0, -1).Value Then
Application.Undo
MsgBox "Value in column B must be lower than value in column A", , "Invalid Entry"
End If
End Sub

This gives you some idea of what you can do, the opportunities are endless!

Regards,
Dax.

Thanks, I'll have a look.