After getting my newly-Debianized laptop connected via wifi to the internet, I edited the file /etc/apt/sources.list
so that it read as follows
#deb file:///cdrom/ sarge main
# deb cdrom:[Debian GNU/Linux 3.1 r1 _Sarge_ - Official i386 Binary-3 (20051220)]/ unstable contrib main
# deb cdrom:[Debian GNU/Linux 3.1 r1 _Sarge_ - Official i386 Binary-2 (20051220)]/ unstable main
# deb cdrom:[Debian GNU/Linux 3.1 r1 _Sarge_ - Official i386 Binary-1 (20051220)]/ unstable contrib main
deb http://security.debian.org/ stable/updates main contrib
deb http://ftp.us.debian.org/debian/ sarge main contrib non-free
The pound sign before the lines beginning “deb cd-rom” causes apt-get update to ignore them. The last line tells apt-get update to fetch a list of sofware packages from the named website. After making the changes to /etc/apt/sources.list, I ran
apt-get update
apt-get upgrade (to upgrade all the software packages to the latest versions in the Sarge distribution).
But upon running the last command, I got a message that 20 packages were being “held back” (no reason given). Going into the aptitude program (aptitude is a nicer user interface for apt-get), it turns out that these packages were broken due to missing dependencies (i.e., missing programs or libraries, or obsolete versions of either). But Sarge is an old distribution (current stable distro is Etch), so why would these dependency conflicts have occurred?
Long story short: commenting out the line about the security updates, then re-running apt-get update; apt-get upgrade completely solved the problem.