The malicious-ad market is considered a multibillion dollar business. So far, though, the fight against it has proven to be difficult. Microsoft over ad-based malware last year, but the problem is widespread enough that lawsuits seem to do little in the way of curbing it. In the new case, Microsoft isn't even sure exactly whom it's going after.

"Although we don't yet know the names of the specific individuals behind these acts, we are filing these cases to help uncover the people responsible and prevent them from continuing their exploits," Cranton explains.

General caution is the best way to keep yourself protected: Don't click onto Web-based pop-up boxes, even if they display alarming messages about your computer; make sure your own virus protection software is in place and up-to-date; and never give out your personal information while surfing the Web, unless you know exactly where it's going.