I had this problem and it actually wasn't a "virus" in the sense that it was malicious software installed unknowingly. Adaware, which I have used forever, has decided to basically become a full service antivirus. Well, it's Real Time Protection function is exceedingly "protective." So much so that it was blocking regular websites, including CC. It took a while to figure out, but Adaware was causing lots and lots of problems on my machine. I couldn't (and still can't) find any traces of malicious code, though.