Columba has a number of command line options which may be handy for you. You can find the full list by running columba --help.
To start Columba and begin composing a message to a email address: columba --mailurl peter@somewhere.com
To make your web browser use Columba as default email client, enter columba --mailurl %s as the email handler in your web browser. When using the windows installer you do not have to do this manually. Columba will automatically ask you if you want it to make it the system default email client.