Is wyUpdate able to INSTALL the update in a windows xp/vista/7 computer without have admin privileges?
Yes, but for wyUpdate to install updates without requiring (and prompting for) admin privileges then your app must be installed to a user-controlled folder (e.g. AppData, Desktop, My Documents, etc.).
In wyBuild 2.6.18 we're making wyUpdate a little more flexible in where it can install without admin privileges. For instance if you're installing to a folder that isn't in AppData, Desktop, etc., but you give a limited user full access to the folder, then they'll be able to update.
In the meantime if you want non-admin users to update your app, put your app in a user controlled folder.