You'll need to save them to your hard drive Peter. Do so by selecting them and the clicking the button of the bottom "Selected Messages" and then choose to download them. Once you get the save prompt and save them, you'll need to click the delete option to remove them.
Also you'll need to do this for the inbox as well as any other folders you wish to archive (such as Sent), all folders count against the quota.
To a small child, the perfect granddad is unafraid of big dogs and fierce storms but absolutely terrified of the word “boo.” – Robert Brault