Suppose we have the customer email id and we want to fetch all details of that customer in Magento, its very easy and you can use below code it will give you all information with the email address which you have added in $email.

$customerExist = Mage::getModel('customer/customer')
->getCollection()
->addAttributeToSelect('*')
->addAttributeToFilter('email', $email )
->getFirstItem();