The Program Files deal is related only to Vista and W7. Anything you modify under the C: (or the drive where the OS is installed) needs specific permissions. Many aspects of poser can fail if said permissions can't be obtained (I've ran into problems with the P8's library and Victoria 4 update tool). So to avoid any headache it's adviced to install poser outside of the program folder or better yet, outside of the C: drive. Either that or turn off windows UAC which is not adviced.

Then again, the previous poster had no problems with P8 or V4, so...

As far as which OS you should use, my personal take is this: Upgrade to W7 if you want, but you wont see any performance boost specifically in poser. The only problem with XP 64bits is the lack of hardware driver support, but if you are only using this PC for poser, that shouldn't be a problem.