To clean up your Ubuntu system it could reasonable to remove old kernels from time to time. For example, if you use a separate /boot partition which have in most cases only some 100MBs space a lot of the kernel images could result in no free disk space.
To list all kernels that are installed in the system use the following command
# dpkg --get-selections | grep linux-image
The output looks like this
linux-image-3.2.0-24-generic-pae install linux-image-3.2.0-25-generic-pae install linux-image-3.2.0-26-generic-pae install linux-image-generic-pae install
To find out what kernel the system is currently using us
# uname -r
To remove on of the old kernels us (DON’T remove the “linux-image-generic” package!)
# sudo aptitude purge [KERNEL-TO-REMOVE]
It is recommended to keep at least one old kernel version installed as backup in case of you have any problems with your current kernel.