Sandra Person Burns used to love browsing and shopping online. Until she realized she was being tracked by software on her computer that she thought she had erased.

Ms. Person Burns, 67, a retired health care executive who lives in Jackson, Miss., said she is wary of online shopping: “Instead of going to Amazon, I’m going to the local bookstore.”

Ms. Person Burns is one of a growing number of consumers who are taking legal action against companies that track computer users’ activity on the Internet. At issue is a little-known piece of computer code placed on hard drives by the Flash program from Adobe when users watch videos on popular Web sites like YouTube and Hulu.

The technology, so-called Flash cookies, is bringing an increasing number of federal lawsuits against media and technology companies and growing criticism from some privacy advocates who say the software may also allow the companies to create detailed profiles of consumers without their knowledge.