The reason is because you don't have the right entry in your sources.list.
Edit your sources.list to look something like this:
Code:
# deb cdrom:[Debian GNU/Linux 5.0.1 _Lenny_Official i386 CD Binary-1 20090413-00:10/ lenny main
deb http://ftp.uk.debian.org/debian/ lenny main contrib non-free
deb-src http://ftp.uk.debian.org/debian/ lenny main contrib non-free
deb http://security.debian.org/ lenny/updates main contrib non-free
deb-src http://security.debian.org/ lenny/updates main contrib non-free
Then do an
apt-get update,then try reinstalling the package.
Obviously you can change the entry for UK to a more local mirror if you prefer.