Just like every good thing is supposed to have a downside to them, so does computers. And the worst nightmare of any computer user is the attack of a virus that slowly and steadily corrupts the internal files beyond repair.
But most viruses are so sly, that until something crashes, you will not have any idea about when and how they crept into your system and stated messing with it. One of the foremost signs that your computer has been compromised is when you find that your PC has slowed down dramatically. If you find your software malfunctioning or your system freezes up, be prepared to shell out some good money to clean and then protect your beloved computer.