T-SQL (SS2K8) :: Select Query - Unique Orders For Date Range?

I have an Orders table which has the following fields:

OrderID (PK, int, auto increment, not null)
CustomerID (FK, int, null)
PaymentDate (datetime, null)
UserID (uniqueidentifier)

(and other irrelevant fields)

Basically, for a specific PaymentDate range (29th July 2014 - 26th August 2014, inclusive) I want to select all orders where they only appear once in the orders table based on the CustomerID, so I only want to know about them if they have a paid order (decided by PaymentDate not being null) in that date range, but also taking into account if they have ever had a paid order outside of that date range. I'll also be joining on to the aspnet_Users table to get the username assigned to that order.