Windows – Install Windows XP/7 without a CD drive when the BIOS does not support booting from USB devices

Burn PLOP boot manager to a cd. This is effectivly a bootloader on a disc that will detect bootable interfaces.

Depends on which model Mac Mini you have. Earlier versions of Intel Macs don't support booting from USB drives as far as I know.

The only other possibility would be your partition structure. Intel Macs use the EFI boot loader, and these prefer the GPT partition structure rather than MBR. If you look closely enough at a drive formatted by Boot Camp Assistant, you'll see a very small FAT32 partition at the beginning of the drive. This is so the partition can be "blessed" and marked as the preferred bootable volume on the machine, so if you restart it automatically boots to that OS. If you cloned the disk from one that you used in a PC, it's most likely formatted MBR.

However, I have verified that it is possible to boot to MBR Windows volumes by holding down the Option key. Only thing is the Windows volumes don't show up until later. However, I'm not sure if this works at all via USB. I would think not.

First step in troubleshooting would be to check and see if your Mac Mini can boot to a USB volume at all... i.e. use Carbon Copy Cloner or SuperDuper to clone your Mac volume to a USB drive and see if you can boot from it.