WD My Book Essential Edition 500 GB Exetrnal Hard... Hello.. i bought this WD 500 GB Exetrnal

Question

Hello.. i bought this WD 500 GB Exetrnal Hard disk USB2.0 few days ago.. USB2.0 And im faceing a problem installing it.. it should install by it self when connecting to the PC as they said... But when i connect it some times says " Uknown Device" Some times "Pritning Support" and im using windows XP.. i had VISTA... but i formated to Windows XP cause they said it will work on XP... i thought that was the problem... When i get in the BIOS page.. and selecting a boot device.. i can see the WD Exetrnal Hard disk ... Any ideas ?.. Best regards -evo