Adware, Spyware, sometimes also called Malware
These are all classes of programs that you didn’t ask for, or didn’t know you asked for and for the most part you don’t want.
If you’re connected to the internet, chances are you’ve accumulated some spyware on your PC. Maybe you clicked on a pop up ad, or maybe you use file sharing software such as Kazaa, or maybe you visited an unscrupulous web site.
Your PC may run more slowly. You may experience random lockups and home page hijacks. You might see a lot of embarrassing pop up ads.
Sometimes, you never know spyware programs are there. They may monitor your computing habits and send information over the internet without asking, making it easier for others to target you with more popup windows.
Keyloggers can record your every keystroke and share that information with others.
Often spyware is poorly written and causes system lockups and crashes.
When the pop-up advertisements become so frequent that your PC is virtually unusable, you have reached a critical mass of spyware on your machine. You will need to take action to clean house.
Browser Hijackers are a particularly nasty class of Adware. Maybe you installed a “Free(!)” search bar in your browser and now your home page keeps getting changed to something else. No matter how often you change your home page, it keeps getting “hijacked” again. These can be pretty tricky to remove, some are even smart enough to reinstall themselves after you reboot.
Viruses
Usually you have to take some action to infect yourself with a virus. Opening an infected email, running an infected program, opening an infected document, etc.
Sometimes viruses are overtly destructive, they might delete important documents, or render your PC unbootable.
Sometimes they use your PC as a base to launch an attack on another computer in another city or country. The Mydoom virus was designed so that all infected computers simultaneously launched a Distributed Denial Of Service (DDOS) attack on Microsoft and Sun Microsystems on specific dates.
Sometimes, they’re just annoying, reminding you periodically that they’re there and you haven’t gotten rid of them yet.
At worst, destructive viruses can delete important files or share your personal information with others.
Make no mistake, viruses can be extremely dangerous to you, your data and to others.
Worms
These typically infect your PC without any overt action on your part. If you think of your PC as your house, then the worms know where all the unlocked windows are. Most PC users are lax about locking down their systems. Worm authors are expert at finding cracks in Microsoft Windows through which they can sneak using an open network or internet connection.
PC Magazine states that today, an unprotected PC will be attacked within 15 seconds(!) of being connected to the internet.