The first part of this article describes it better then I could:

"Star Wars fans Web hunting for a downloadable game of the sci-fi flick are unknowingly infecting their computers with a worm that gives them rogue Google results authored by hackers.

Detected in the wild last week, the P2Load.A worm lies in wait in the free version of “Knights of the Old Republic II,” and spreads through the peer-to-peer file sharing programs Shareaza and Imesh.

Security experts at Panda Software rate the potential damage of the infection at four stars, the highest level, based on the worm’s two attacking features both enabled through modifying the HOSTS file on a user’s computer."

Two words people...  VIRUS SCAN.