There are a number of ways to achieve this requirement. You could create a custom lead-to-product relationship and then associate a lead with an existing product. But personally, I would skip using leads and just create an account for all existing and prospective customers.
For your accounts, you could do the same thing: create a customer lead-to-product relationship, but you might be better with a link entity so that you can capture more attributes about the customers' interest. This would be a custom entity, 'Product Interest', associated with an account (N:1) and a product (N:1) plus some custom attributes such as Owner, Interested On, Interest Level, etc. Or you could have five checkboxes on the account form, or create a marketing list for each product and add the account to the relevant lists. Or you could just an opportunity when an account expresses interest in a product -- after all that's exactly what a qualified sales opportunity really is.
The most appropriate solution for you will depend on the detail of your business requirement and what you want to do with the interests once they are stored in CRM.