Plz help me to replace vlookup formula with a macro

Plz help me to replace this
formula with a macro
=IF(A1="","",IF(ISNA(VLOOKUP
(A1,Sheet2!
A:B,2,0)),A1,(VLOOKUP
(A1,SHEET2!A:B,2,0))

What is the purpose behind it being a macro? Is it to only display the value? How would you output the result, into a cell, message box, or what?

By this formula i change old account into new account no.
plz help to create a macro code

I have an excel file in which
sheet 2's A column contains
Old account number B column
contains New account number.
I want that
when i enter an old no in
column A then new no show in
column B of sheet
2.
in this case i need Excel
experts help
please tell me how can i create
a VBA code to do this.
But i dont want to use a Vlook
up formula.
thanks in advance