grams, I use ATT and sometimes the same thing happens for me when I open Internet Explorer browser to ATT's own home page. It's a message about a security certificate for some page that "wants" to open up. Does the message about ordering from Amazon.Com through the link that pops-up here show-up? Internet Explorer is not recognizing some pop-up as secure. I suggest that you just ignore it, or that you try another browser, like Mozilla's Firefox. It's not going to do any damage to your computer or operating software.
If you're not real computer savvy do you have a friend or family member who can help you do this? If you download a new browser like Firefox you'll be hit with questions about setting up a home page and transferring Favorites from Internet Explorer, etc. It's not a problem for someone with experience with such things.