First goto synaptic package manager and go to Settings - preferences - Files-
Tempory files and select "Leave all downloaded packages in the cache ".
Then go to the cache folder /var/cache/apt/archives.
Here you find all applications packages downloaded with apt.
Now you can copy them to another partition or burn them on a cd disk.
So you don't have to download them again after a complete reinstall.
As a newbie to linux I search for days to find this !!!!!!!