|"It will be slower and unreliable"|
To expand on the speed comment, consider the differences in theoretical data transfer rates:
USB 1.0 = 1.5 Mbit/s (0.19MB/s)
USB 1.1 = 12 Mbit/s (1.5MB/s)
USB 2.0 = 480 Mbit/s (60 MB/s)
ATA 100 = 800 Mbit/s (100 MB/s)
SATA 1 = 1200 Mbit/s (150 MB/s)
SATA 2 = 2400 Mbit/s (300 MB/s)
Real world transfer rates will be much slower. If you have an older motherboard that doesn't support USB 2.0, don't waste your time. It probably wouldn't support booting from a USB device anyway.
EDIT: after re-reading your question, I'm wondering what it is you want to do? Do you actually want to run Windows from an ext USB HDD or do you just want to install Windows to the ext HDD & then transfer it into a computer?