
Improper use of "free" file-sharing programs can result in potential legal liability if the files being shared - music, movies, software - are copyrighted.
Infection by worms, viruses, trojans and other "malware" that can allow your computer to be controlled by hackers from anywhere in the world.
General computer slowness and privacy issues
How do many web sites offering "free" software support their development costs? Through advertising income from marketing firms that attach spyware programs to your system which piggyback on the "free" software installation! This software often tracks the sites you visit and sends this information to spammers all over the world.
Spyware can grab personal data you enter in forms and send it all over the world.
Many spyware programs are "self-updaters", i.e. they can change their function on your computer at the whim of the designer, without your knowledge. Improper removal of spyware may damage your network connection and render it unusable.
Act Responsibly
OnGuard Online, U.S. Department of Homeland Security