I've enabled single-click for opening files, folder etc. It is working normally inside a folder but i still need to double-click on files/folder on desktop. Please advice.
Desktop icons are displayed by xfce4-desktop, not by the Thunar file manager, unlike the gnome and unity desktops. This is due to change in future releases, but for now there is nothing you can do.
If you update to Xubuntu 12.04, the option to single click on desktop launchers / files is now there. It has been back-ported from the new version of XFCE, which will be available in Xubuntu 12.10.