I'm trying to install a new OS onto a computer from a bootable USB stick. I previously installed Ubuntu Linux and it was a piece of cake -- I downloaded the ISO image, used UNetbootin to copy it to the USB drive and make it bootable, and that was that.

Now, however, no matter what I try, I can't make the same USB drive bootable again! I've tried formatting it as FAT32 and NTFS. I've tried several different Linux distributions and Windows 7. I've tried using UNetbootin, Windows 7 USB Download Tool, WinToFlash, and manually making it bootable with diskpart/bootsect/bootrec. (Yes, I've tried bootsect /nt60 x: /force.)

None of this seems to be working! When I try to boot from the drive, the machine reads from it (I can see the drive's LED blinking) and then gives me the same "Insert system disk and press Enter" message. (I've disabled booting from the hard drive.)

Am I missing something I need to do to make the USB drive bootable again?

The USB drive in question is a SanDisk Cruzer 8GB SDCZ6. The computer I'm working on is running Windows Vista SP1.