Restoring jailbroken ipod help!!!!?

i tried restoring my ipod to update to software 4 and i cant restore it, wen i click restore, it ses its not legible or something and it ses it cant restore and i tried restoring it in DFU mode and same thing it said it wasnt legible and cannot restore... what do i do? i really want to update and ive been told in order for an update if uve got a jailbroken ipod u must restore first... PLEASE HELP PLEASE!!!!!!!