recently i have made several bootable USB drives with a variety of linux distributions ranging from Ubuntu to Backtrack using the Unetbootin installer. lately however, the devices created stopped working altogether, and will not boot. i tried using a DVD instead, but it would not boot, and instead showed a black screen saying that it could not find the kernel, and when i typed it in, the computer said the boot kernel is invalid. i don't see how this could be possible, having downloaded the OS from the websites with no tampering whatsoever. When i use a USB drive, the machine just flashes "invalid operating system" or something of the sort (goes away too fast to see). What is going on here?
This isn't a complete, definite answer, but I know that Unetbootin wasn't able to create valid Live-USB installs for Ubuntu recently (I think around 11.04). That might be continuing into 12.04, and perhaps it could be a more far-reaching bug.
How did you make the DVDs? You might want to try burning them again at a lower speed; sometimes you can get unreported errors during creation.
It might also be a good idea to go to pendrivelinux.com (or use the built-in USB creators of many OSes) or some other site to get an alternative USB-creator.
Maybe you could try updating your BIOS if nothing else seems to work.
UPDATE: Ridiculous that I seem to have missed this, but you have tried MD5 checking, right? The most likely thing is that your files are corrupted.
Maybe you have changed HDD mode from SATA mode or IDE mode to AHCI ..
This is most common mistake people do .
If not set bios to its defaults and retry.