Bootable windows 7 install USB stick not detected as boot device

For a while now I have had a copy of windows 7 ult. On a flash drive, and have booted from it multiple times on other PCs, but I just tried it on another pc, and it doesn't work.

I sort through BIOS so I can change boot order and install windows 7 from the USB stick, but it isn't found as a boot device. it is, however, detected as a hard drive.

I am a little confused, because i just tried it on my personal PC and it was detected just fine as a boot device.

any help would be awesome! thanks!