Windows 2000...? At least get XP. Either way, you would install it the same way you would install any OS. The external hard drive would have to be recognizable by the BIOS of you computer. So essentially, you should use a CD/DVD or flash drive if you can. Most external hard drives have software that allows/blocks read/write access to the drive. So without an OS to use the software, your computer won't recognize most external HDD's.
For those about to hack, I salute you.