Linux Mint 14.1 not bootable because of “secure boot”

Question

I'm trying to install Linux Mint 14.1 on my Lenovo B570, but when I try to boot the thumb drive, it displays a message: "Secure boot is not enabled" and then goes black screen. I read lots of posts on the subject, but there is one difference from other people's problems. I have Windows 7 installed (since over a year ago) and around 5 months ago I installed Kubuntu 12.10 without any problem (it was even the same flash drive). Now I'm facing the secure boot problem and I don't know what to do.