The "This computer is not authorized..." message is an indication that the "cookie" stored on your computer that identifies it as being authorized has been erased. Note that this cookie is actually stored in the browser, not the computer, so using a new browser (or updating an old one) can cause this cookie to disappear.
Security settings in a browser affect how long cookies are stored there. To change your security settings in FireFox (our recommended browser), start by pressing "ALT-T" (PC) or "Command-T" (Mac) or clicking the Tools link, if it is present. Next choose "Options" then "Privacy" and make sure you have selected "FireFox will....Remember History."
Yet another way that cookies can disappear is by being erased by your virus scanner software. I can't give specific instructions on how to tell this software to stop erasing the cookies, since there are so many different brands of virus scanners, but poking around in the settings of your virus scanner can result in finding how to change the cookie deletion settings there.