Major websites such as MSN.com and Hulu.com have been tracking people's online activities using powerful new methods that are almost impossible for computer users to detect, new research shows.

The new techniques, which are legal, reach beyond the traditional "cookie," a small file that websites routinely install on users' computers to help track their activities online. Hulu and MSN were installing files known as "supercookies," which are capable of re-creating users' profiles after people deleted regular cookies, according...