I've messed up with repos. How do I get the default list of repos back? What about the keys I've probably have installed - but would not be used - if I'd roll back to the default repo list?

Edit:

Here's the full story. On updating to Kubuntu 12.04 - virtuoso-t started to give some error (it kubuntu tells me that is crashes on start up). I thought - re-installation would solve the problem. So I made a complete re-installation of nepomuk. I didn't notice - but it has also removed my kde! So on restart I've login to Gnome session. So I get to the Internet, opened a synaptic - and tried to install kubuntu-desktop. But it turns out to be a some dependency problem - nobody were to provide a plasma-desktop libs and so on. So I used the ubuntu repos generator (link is in the answers - I always get there by googling). But it didn't help. Finally I found a machine with Kubuntu 11.10 - I've started an update process - have waited until /etc/apt got updated, took it to my machine - no luck again: dependecy problems. Finally adding:

sudo add-apt-repository ppa:kubuntu-ppa/ppa 

sudo add-apt-repository ppa:kubuntu-ppa/backports 

sudo add-apt-repository ppa:kubuntu-ppa/beta 

have solved it all.