I've downloaded an ISO file of Ubuntu 12.04 and created a Live USB but when I try to boot from USB this Error comes up.
What should I Do?
The problem is usually not with the ISO or with the USB creator but with the BIOS itself.
To fix this problem, go to Bios Boot Menu and search for USB Mass Storage Emulation type.
Most sources on the net seem to blame the BIOS for this, but in my case I was able to solve the problem by formatting the USB drive to ext4 rather than the usually suggested fat32. Even if this doesn't work, perhaps try some different formats (ext2, ntfs, whatever)