If you have a nicely running Ubuntu system with all your favourite packages installed, it might not seem like a good idea to start all over or to replace your hard drive. But here's a tip: To save a list of all packages that are currently installed! This should be a part of any good backup.
apt-cache --installed pkgnames
Save them in a file:
apt-cache --installed pkgnames > installed.packages.lst
To install all packages in a file:
sudo apt-get install `cat installed.packages.lst`