1) I'm aware ofusers using Win2003 with no problems; we aren't OS specific in any of our calls.
2) PhotoPost is generally geared toward public galleries with support for individual private galleries; you can configure it as you want and users can simply select to make their albums private.
3) We do not have a PayPal Integration at this time.
4) We are working with a vendor on providing this options.
Please do not PM me for support or sales questions. Thank you for your understanding.