You are running Debian stable, because you prefer the Debian stable tree. It runs great, there is just one problem: the software is a little bit outdated compared to other distributions. This is where backports come in.
Backports are recompiled packages from testing (mostly) and unstable (in a few cases only, e.g. security updates) in a stable environment so that they will run without new libraries (whenever it is possible) on a Debian stable distribution. It is recommended to select single backports which fit your needs, and not to use all available backports.
/etc/apt/sources.listにcdnを追記する。
deb cdrom...は先頭に#をつけてコメントアウトする。
#deb cdrom:[Debian GNU/Linux 5.0.6 _Lenny_ - Official i386 DVD Binary-1 20100904-19:12]/ lenny contrib main
deb http://cdn.debian.or.jp/debian/ stable main contrib non-free
deb-src http://cdn.debian.or.jp/debian/ stable main contrib non-free