Imagine if you were able to obtain a $1500 piece of software for free. Sounds good, right? The problem is, the software has been hacked and modified and when you download it, it is not with the permission of the company who made it. Software piracy is a big problem for companies. One thing you can add to that is pirated software may have more than you bargained for. Pirated software, music, and movies may contain a viruses or something worse. The simple fact is, except in a rare few cases, getting something that costs money for free or a greatly reduced price is probably illegal in some way and if it is a computer file it has a good chance of containing a virus.
Here is a real life example, one that I have seen. Someone downloads a movie through Limewire. This movie is copyrighted, so it is illegal for it to be on Limewire to begin with. Of course, some things on Limewire are legal, so Limewire isn't the problem. But the illegal movie is the problem. A person downloads the movie, but the movie has a virus hidden in it. The virus is designed to exploit common programs used to watch video files on a computer. You watch the movie, the virus comes alive, replicates itself onto other movie files, and does some other malicious activity to your computer. Usually it downloads some very nasty malware. These types of viruses generally affect Windows computers as much as Macs.
Another example. Windows 7 just came out. And you want a copy. But uh-oh, you don't have $150 to get one. So you download it. Windows requires a code of 25 letters and numbers in groups of 5. You did not purchase the software legally, so you have to get a program that generates a code which is called a keygen. Obviously, keygen programs are helping you to do something illegal, so they won't be found at a safe website. You download a keygen, and in the process get a ton of malware onto your computer. Or, you download a copy of Windows 7 that is hacked so as to avoid the required Microsoft activation. But uh-oh, it has a virus in it and just by trying to install it, your computer will be riddled with many nasty programs.
Beware. Free in cost does not mean it is free of problem. The idea you need to remember is that if it costs money and you get it for free or for a price much lower than everyone else, then it is probably illegal.