What disk are you trying to install from? Normally you will simply boot from the CD and follow the install instructions. You do not have to repartition, just install on the same partition where your current installation is located. You will choose manual partitioning to do this. You must be very careful here to make sure that manual partitioning is chosen and NOT AUTOMATIC partitioning or Ubuntu will try to take over the entire disk.
If you are reinstalling, Grub should not be involved in the re-install, as grub will be re-installed as well.