You can put Debian on a memory stick (keydrive) and boot it directly from the memory stick.
The DebianInstaller can both boot from USB keydrives, and install to them. It allows you to write the installer to one keychain, plug it in, plug in a second that will hold the target system, and do a straightforward installation to it without touching any local disk. (Alternatively, you can boot the installer from a CD.)
See also: Keydrive