Paypal Payment Standard doesn't require to be registered to paypal to perform payment.
However I've read on a blog that the second time you proceed, it recognize your credit card and ask you to register to paypal.
Could you give me a 100% sure answer about this ? I really cannot ask customers to create paypal accounts.
Update: I've found the following extract on a website:
Are PayPal accounts required in order to make a purchase or donation? Not any more. It was this way before but PayPal.com changed the policy. You may make ONE non-recurring transaction without opening a PayPal account however, PayPal creates a temporary PayPal account with credit card number used. If you use the same credit card again with so many months, PayPal will ask to open and verify an account.
I was wondering how often it should be a payment done to see paypal asking to make an account. For example, let's say that I'm performing the second payment after 2 weeks. Is it going to ask me for a paypal account ?