11.10 - How do I return to a previous kernel?

I downloaded and installed the 3.0.0-21 update to the kernel. It caused several problems including:

1. Firefox not asking for a folder before placing a URL into bookmarks


2. Ubuntu 11.10 not automatically opening to the desktop (I still haven't figuring out a viable work around).

This may be an inability to hibernate and reinstall, but I believe I also had problems after shutdown and booting.

I have tried to learn how to return to the former 3.0.0-19, but I can't find anything to teach me. I would like to know how to return to the -19 configuration or how to reinstall the -21 configuration.