I just went ahead and did a completely new install of 12.04 last night after idiotically and automatically saying yes to one of those "partial upgrade" prompts in the software center or upgrade manager. Long story short, it ruined my system as predictable and I didn't want to go through and reinstall all the missing vital packages, so I just threw the whole broken system out.
The next morning (it decided to take a number of hours to install the new OS and I fell asleep), I re-started and attempted to load some software that I'd had before. Seconds, later, it seemed I'd been transported back to last night with a useless system. First, the universe repository was not loaded and errored out if I attempted to add it. Then, package manager started throwing up a million error and many of my system programs won't open. Here's what the terminal says when I attempt to update:
And here's what happens when I try to start Update Manager:
I'm using the 64-bit install on an HP Pavilion dm1-4010us with an AMD E2 processor.