Thousands of Internet users who installed popular software for sharing music and other computer files also unwittingly accepted a program that tracked their Web surfing habits.

The companies that produce LimeWire, Grokster and KaZaA have since posted new versions of their software, absent the tracking program. LimeWire's maker also issued an apology.

The tracking program in question links Internet users to an online sweepstakes game, ClickTillUWin. Players pick numbers and win cash prizes based on results in the Pennsylvania Lottery.

The game is free, though users must first view an advertisement. The program collects information about sites visited over the past two days in order to better target ads.

These types of programs exist elsewhere on the Internet, and what prompted concerns in this case was the lack of disclosure or an option to reject it.

It's unclear who's to blame.

Mike Calderone, president of Strategic Advertising Services Inc., which runs the ClickTillUWin game, said outside distributors had been instructed to get users' permission before installing the software — but that was not done.

Greg Bildson, chief technology officer of Lime Wire LLC, said the company was led to believe the program did no more than link to a game, making the permission request unnecessary.

Robert Regular of Cydoor Technologies Inc., which distributed the ClickTillUWin software to the file-sharing companies, said the program wasn't supposed to collect information until users activated it — and had an opportunity to be notified.

Regular said he did not believe deception was intended by any of the parties.

About 150,000 LimeWire users and up to 50,000 Grokster users downloaded ClickTillUWin last month when they obtained new or upgraded versions of the file-sharing software. A few hundred thousand people got it through KaZaA.