Hi,
be also warned that x64 versions of Windows XP are not officially supported for releases prior to v11.0 sp1.
Vista x64 is certified only starting from v.12.0.
Generally speaking, running an Ansys-32 on a 64 bits machine MAY work (I never tried) though it is uncertified. You should get the advantage that you would be able to adress the full 4 GB allowed by the 32 bits, instead of less than 3 under x86 versions of XP / Vista.
x64 version of Ansys is another kernel, it resides on another DVD. Phenomenal when you have enough memory to run your entire problem incore with the direct sparse solver.
Regards