If auditing is not enabled then you cannot track who deleted the files. You need to have auditing - or some form of logging - enabled at the time when the files were deleted, because retrospectively attempting to search a log of an event which does not exist will prove to be impossible.
In an XP Pro workgroup, I doubt proper security, usernames and passwords are being used anyway; rather, I would guess all users access this XP Pro machine using one user account, which would mean any log would prove unuseful anyway.
-tigermatt
