Excel 2007 : Return value if two conditions are met

Hi,

I'm working with a costumer database and want to add new costumers to a column which consists of current and expired ID-numbers. I have a workfile with the summary list in column A and costumers for march 2010 in column B. The problem is that I don't want old costumers to appear as new in the summary list if they have been costumers before. This occurs if they have been costumers, then had a break, and now became costumers again.

So lets say I have the summary of existing and expired ID-numbers (until March 2010) in A1:A50, and the costumers in March 2010 in B1:B30. Column C is made to detect new costumers in March by comparing with February. New costumers are assigned with the label "NEW" in column C.

What I'm trying to do is to add the ID-numbers that meet the following conditions:

1) They are new (assigned with "NEW" in column C)
2) Are not already in column A

How can I build a formula that meets these conditions?

My apologies if the explanation is hard to understand.

Unfortunately there is no help in attaching the file because my excel version is in Norwegian..

Thanks in advance.