You can boot Windows from a USB drive, whether external hard drive or
flash drive.

And there are also adapters that will let you plug a flash drive into an IDE
port. Just plug the adapter into the motherboard's IDE port and then a
flash drive into the adapter. The computer sees it as just another drive.
Granted, the OS won't generally see it as removable so it's not a typical
way to use a flash memory card. But it's a great way to add a
no-moving-parts drive to a system that doesn't otherwise directly support
that sort of media. You can also use those IBM/Hitachi CF-card MicroDrive
cards if you want actual rotating memory (as 'unmanaged' flash has it's
share of long term read/write issues).