I have been playing with package management by adding sources from older Ubuntu releases in order to get older versions of some software (e.g. PHP). Unfortunately at some point I must have overdid it as now every attempt to use apt-get or synaptic ends in an error message being displayed saying that there is no candidate available to install.
I would like to start fresh - remove all the installed packages and added sources. Is there a quick way to do this, or do I need to reinstall the OS?