Hello I have made a bootable USB drive(containing Ubuntu 12) using YUMI in Windows. Then I tried to boot from the USB using F12 key and properly identify the pen drive and enter. After few seconds of waiting it boots from Windows 7 (as I have Windows 7 installed on my computer). When that method failed , I make rearrangement in the boot option using F2 key and bring USB boot option at the top, but that also didn't help.
Just two months ago also it was working in my computer.
Note:I tried to boot on my friend's computer using same pen drive, there it worked perfectly.