That was a pita to fix the other day - reimaged my in-laws (70+ years old) computer, because they apparently fall for every popup that says "omg, virus! Click here to install our software to fix your computer" and "I've got to have comet cursor! I don't care what kind of software it is, I like their smiley faces the best." Anyways, WinXP, with IE6. Any attempt to connect to Microsoft to update IE was met with "your computer is soooo outdated, go buy a new one." I couldn't avoid that at MS. I couldn't install Firefox, because the software was too out of date to download it. I succeeded with Chrome, and got that message for all sorts of sites. Then, Firefox from Chrome (that was a pita; I got that message for FF as well.) Then, from Firefox, a download of service pack 3.
Anyway - it's not necessarily because of a virus; as I had that problem behind a firewall with a completely newly formatted computer.
(Oh, and the in-laws are thrilled that their computer does Facebook and their email once again - that's all they use it for, and it's more than adequate for that purpose.)