This should get you started.

You could attach some code to the Worksheet Change that checks to see
if the two cells are equal and if so run some code. This method will
check every time something is entered. I'm sure it needs to be
modified for you needs but should give you a starting point.

Private Sub Worksheet_Change(ByVal Target As Range)
Set Target = Sheets("Sheet1").Range("A1:B1")
If Sheets("Sheet1").Range("A1") = Sheets("Sheet1").Range("B1") Then
MsgBox "(put code here)"
End If
End Sub

- John