I created a live USB of windows 7 with unetbootin. I'm running Ubuntu 12.04. I have a Lenovo z580 intel core i7 with 8gb ram. My computer won't boot from USB. USB is first in the boot order. My stick is flagged as boot in gparted. I tried booting from usb via system setup from the grub menu. It just sends me back to grub. BTW the usb stick is formatted fat32 (as per a recommendation that unetbootin will fail with NTFS) however I also tried with it formatted to NTFS and it also fails to boot from USB.
I think your USB MBR (Master Boot Record) has corrupted. Open Terminal and type these commands:
sudo apt-get install mbr
fdisk -l (to find out which device is your flash drive)
install-mbr /dev/sdx (replacing x with your flash device)
and then create the live USB again.