I have a Windows Vista PC with to internal hard disks.

I want to try Leopard on my PC but I don't want to make any changes in my Vista installation.

I want to know if this is possible:

Install Leopard on a external USB hard disk (all the entire hard disk will be for Leopard). Is thar possible without install anything in the internal hard drives?

In the boot process.....how can I select which OS i want to use? I'd like to make it without using any bootloader loaded in the internal hard drives.

Could it be selecting booting options in the BIOS? (primary hard disk or USB hard disk?

Thanks in advance.