You could compare the list of installed package with the ".manifest" file of your distribution,

For instance the manifest of 13.04/i386 can be found here :

http://mirrors.mit.edu/ubuntu-releases/13.04/ubuntu-13.04-desktop-i386.manifest

To get your installed packages :

#aptitude search ~i !~M

Regarding the customization you could have made there is a tool that can helps you to compare the modified configuration files against the package checksum informations : debsums

#debsums -ce 

(-e to check only the configuration files, -c only the changed files)

Edit :

the packages installed during the year are logged in /var/log/apt. Older history logs are deleted by logrotate. I made that script to get the installed package (take care not to modify the awk patterns when copying).

# cd /var/opt/log

#(zcat $(ls -rt history*gz); cat history.log ) | awk '

 /^Commandline: (apt-get install|synaptic|aptitude)/{ 

         cmdl=$0 

         getline

         if(/^Install|^Remove/) {

            print cmdl

            print

         }

  }' | less