MY Ubuntu 8.04 upgrade was messed up a bit.Since I couldn't update my installation,I updated the packages and it seemed to work.A few days ago,I was wondering why I still had Firefox 3 Beta 5.I checked my software sources to see that the sources were outdated.I was getting updates from the gutsy repositories.
Ankur suggested me to replace my 'sources.list' file to sort out the problem.I did so and it worked.Here's a guide for noobs on how to do so-
1)Boot from the Ubuntu 8.04 live CD.Select Try Ubuntu without making any changes to your computer.After the live session loads completely,open a terminal window(Applications->Accessories->Terminal).
2)Run nautilus in super user mode by running
Kubuntu users type 'sudo konqueror' and xubuntu users 'sudo thunar'.
3)Navigate to /etc/apt.Copy 'sources.list'.Then navigate to the partition with Ubuntu installed.Replace /etc/apt/sources.list with the file you just copied.
Now boot Ubuntu and go to System->Administration->Software Sources.Move to the 'Updates' tab.Make sure that 'hardy-security' and 'hardy-updates' are checked and 'hardy-proposed' and 'hardy-backports' are unchecked.
This solved the problem for me and I find no reason why it shouldn't work for anyone else.I hope this solved your problem.