The driver for an Apple Laserwriter 12/640PS is built into Windows XP - there is nothing to download or install. If you are using this printer on a network, you do need to install the Windows LPR print service - an optionally installable software component of Windows XP. See
here for example.
Reference: A list of printer drivers that are included in Windows XP
Reference: A list of printer drivers that are included in Windows XP