Should show the email addy, is it a standard , Ie Bt, Vgn media Tiscali or a Hotmail A/c .. Chances are they have just paid the wrong email address, But I Would leave it, do not refund and let the payer request paypal return the money which they will deduct from the account.
Have you sold anything for that amount as the buyer may have got someone to trf the funds to you, although I would have thought that they would put some info in the comments section