If you see this message displayed in IE7 for internal websites (or published sites such as OWA or Team Services) this is because (1) the computer you are using doesn't trust the issuing authority of the website's certificate, or (2) you are using the internal name and the certificate has the full DNS or external name. In either case, it's OK to continue by clicking on the "Continue to this website".
Long term the fix is either to (1) download the certificate authority's root certificate and add it to the machine's list of trusted root authorities, or (2) use the correct name of the site, as on the certificate.
Of course, there's always a 3rd reason – the certificate may have expired, in which case it will need renewing on the server.
if you need assistance with certficates or need to purchase a new certificate, please see our