Good afternoon

I am getting the following error
Package in inconsistent state

The package 'linux-headers-3.0.0-20' is in an inconsistent state and needs to be reinstalled, but no archive can be found for it. Please reinstall the package manually or remove it from the system.

I cannot find a copy of 30.0.0-20 anywhere. I can start my system via previous version of linux, 30.0.0-17

I would like to know how to make the system start automatically via 30.0.0-17, and delete references to 30.0.0-20. I can the upgrade to 12.10, currently 11.04.

Does any know where I can find a good guide on how to do this please