If you are using OneCare, this is not possible with the OneCare printer sharing feature. If you are not using OneCare, you are off topic for this forum and should head over here for more help in the Vista forums -
http://social.answers.microsoft.com/Forums/en-US/categoriesBasically, if you need to share the printer, the drivers must be installed on the XP PC manually from disc or download from HP as the 64 bit PC cannot share the drivers with the 32 bit PC.
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator