dude,they are stored in /var/cache/apt/archives directory-may be u can backup.but remember u need to "reload" option of synaptic or use "apt-get update" from terminal to get newer packages(softwares)
do try reading:*www.justlinux.com/nhf/Distribution_Specific/Debian_GNULinux/Debian__Using_Apt.html
[SIZE=-1]www.debianadmin.com[/SIZE]
*www.debian.org/doc/manuals/apt-howto/
*ubuntuguide.org