That's the issue. You can't directly write to the boot sector of your USB drive (which UNetbootin needs to do to install the syslinux bootloader to the USB drive, ie make it bootable) without administrator privileges, thus this isn't a solvable problem due to the operating system's security model.
However you said you were able to "pre-format the USB stick from a running Live CD" so you can try running UNetbootin from within the live CD environment on your notebook because in that case you do have the necessary administrator priviledges via sudo.
Windows user:
common user lacking admin privs
That's the issue. You can't directly write to the boot sector of your USB drive (which UNetbootin needs to do to install the syslinux bootloader to the USB drive, ie make it bootable) without administrator privileges, thus this isn't a solvable problem due to the operating system's security model.
However you said you were able to "pre-format the USB stick from a running Live CD" so you can try running UNetbootin from within the live CD environment on your notebook because in that case you do have the necessary administrator priviledges via sudo.