This is a crude answer but it works. Print a list of your installed programs, things you've installed, not a full package list. Backup your /home/ and reinstall.
To get a list of installed packages
dpkg --get-selections > installed-software
This creates a file installed-software listing the software installed on your computer. You can compare that with the list of files installed by default to find what has been added if you want.
Any version of Ubuntu has a manifest , where all the default pre-installed packages listed.
For 12.04 LTS this list is here => http://nl.releases.ubuntu.com/precise/
Scroll down the page and you will see a file ends with .manifest
Depends on what version of Ubuntu have installed (i386=32bit or amd64=64bit) download the manifest and open it with Libreoffice to read it.