The grub boot menu in Ubuntu can become pretty crowded over time with older version of the kernel. When you update Ubuntu, older version are not automatically cleaned out of the grub boot menu. But if you want, it's pretty easy to clean out the older, or unused, entries from the grub boot menu.

The entries that are shown in the grub boot menu are contained in the file /boot/grub/menu.lst. To clean up grub, you just need to edit this file and remove the old entries. When editing this file, you need to do this as root. You can either use sudo on the command line to edit the file, or you can use gksu to edit this in your desktop environment. As an example, from the command line you could type: sudo vim /boot/grub/menu.lst. From the desktop you could press Alt + F2 to get the Run menu and then type: gksu gedit /boot/grub/menu.lst.

Either way, once you are editing the file, you want to look for the line that reads:

## ## End Defaults Options ##

Once you find this line, there will be sections that follow and each section represents one line in the grub boot menu. It's very easy to determine which section represents which line in the boot menu by looking at the title attribute. Once you decided which sections you want to remove from the grub boot menu, delete them and save the file. Next time you boot up, you'll have a nice clean screen in grub.