I recently upgraded from Maveric(10.10) to 12.04 (by wiping out Maveric from my system completely) I was using a pen drive, so it messed up my grub. I finally managed to reinstall the grub on the machine and now it boots fine.
Now,I can't download anything from the Software-center or manually. I followed some advice on the forums that included purging the software centre, but once i tried to reinstall, it refused. I checked my software sources they all seem ok. I'm set to main server. I can't install anything.