Martin:

I know an app that can help assist you on this. You can download it from
http://www.tairasoft.com/download/DataMinerBeta.zip

I use this tool to extract certain portion of the data from the worksheets.
The initial step requires you to convert each of your data worksheet into csv
file. After that the process is quite simple.

If you are planning to use this tool then you need to import two of your
data lists. From that point you can run a simple query to both tables. You
can get the result in which list of new accounts are not already on the
database and are in the database.

Regards,
-Kris