How To Compare Two Column or Rown in Excel

What happen if you have two columns or rows consisting of thousand of name or number that you want to compare? For example, last month I had more than 1000 product numbers from customers. I want to know which customers are disconnected from this month. So here is what I gonna do to make it less miserable.

1. Excel 2003 (Vlookup)

From now, wherever the zero (0) number is, I can find out which customers are missing in this month.

Have fun!