problem with accounts and customer login

[Jayne]

I have a problem with the cutomer log in my customer created an account and bought something now she is trying to log in and it says she is not registered so she tryed to open a new account and it says her email address is already registered, and it is I have all her details in the orders section but nothing in the cusomers registered section can anyone tell me why this is I have also had problems with people logging into there account and finding some one elses name there along side theres I'm baffled please help
many thanks
Jayne