new 2.5 HDD in encloser

Hi
I have recently bought a new WD120 2.5 HDD and placed it in a A Plus USB encloser. When I plug it into my computer running Winxp Home it Identifies that a USB Mass Storage is there but does not assign a drive letter and it does not appear in the list under my computer. I beleive that the drive needs to be formated but how?