simple Windows XP support for datadir

Windows XP sadly does not have a simplistic LOCALAPPDATA env variable to use. It only has APPDATA and that should be used instead.

BTW: Windows XP does have a equivalent folder to LOCALAPPDATA as in Windows Vista and up, but its localized into the target OS language and thus difficult to code for. I think the simple APPDATA fallback is sufficient for most use-cases though.