a) You might have problems with device drivers, many (usually older) device drivers don't work under 64 bit XP. Be especially careful with printers.

b) I don't know, for 32 bit stuff I use my notebook.

c) I think you'd have to re-install. If I understand it correctly, at install time Vue decides which version to install.

d) In my experience, Poser 7 works as well under 64 bit XP as it does under  the 32 bit version.