HOW CAN I GET XP TO RECOGNISE EXTERNAL HDD?

Hi everyone,ive recently upgraded my ps3 harddrive,the one ive taken out of my ps3 is to be used as an external harddrive,ive piut it in an enclosure so i can connect it up to my desktop pc via the usb,when i did this,for some reason my pc doesnt even know ive got the harddrive hooked up to it for some reason,the pc is on windows xp,any ideas how i can get it to see the external harddrive,thanks for your help