Comment 19 for bug 131134

I tried this and it worked:

Problems Encountered And Their Solution

If while downloading the packages, tasksel unexpectedly exits with an error – tasksel: aptitude failed (100), do not despair. It is because the package database of your Ubuntu installation is old. Doing a sudo apt-get update will resolve the error and tasksel will be able to continue where it left off.

Ref: http://linuxandfriends.com/2009/12/09/how-to-install-lamp-server-in-ubuntu-linux/.