Obtain a LiveCD distribution, such as Morphix, from
http://www.morphix.org. See Section 4.3.3 for
details of Morphix and Section 3.1 for some
LiveCDs based on Debian GNU/Linux. Boot your computer from this CD and
you will have a running system (but not yet installed). Then ask
Morphix, for example, to install itself. You will end up with an
installed Debian GNU/Linux system. Note that it may have some subtle
differences to a standard Debian install, but it should not be a
problem.