updating website using windows service

Question

Hi

I need to update customers on Site A. Information is extracted from Site B. But we want to be able to update new customers?

not sure how i can do this unless i pass through every customer email and check for the email through the web service method provided by site A? this seems a bit long winded as this is going to be a timed windows service

could i do it any other way?

regards

Tj