Is there a way to reset all packages/sources and start from scratch?

Solution 1:

You can do an installation of Ubuntu over top of an existing installation. You'll lose all of your (non-local [1]) system files and applications, but it will preserve everything in /home.

Select the advanced partitioning option from the menu of either the desktop CD installer or the alternate CD installer. Set the mountpoint of your existing root partition to / and make sure the format box is not checked. Repeat these steps for your home partition, if you have one.

1: Where local system directories would be /usr/src, /usr/local, and /var/local.

Solution 2:

You could always remove all packages (making a few exceptions for apt-get, etc.)

Then run:

sudo apt-get install ubuntu-desktop

This installs the desktop metapackage which has pretty much every other package as a dependency.

Solution 3:

Remove all but the current release of Ubuntu you're running from /etc/apt/sources.list. Then sudo apt-get update; sudo apt-get dist-upgrade. Let me know if you still have trouble.

You may have to reinstall.