Recent papers have reported that a computer connected to the Internet is on average infected within 15 minutes.It also happens if you do not surf dodgy websites...Surfing google or clicking a link is enough to become infected by SPYWARE.
You can easily get worms and trojans opening a wrong mail, maybe coming from a friend.As soon as a trojan gains access to your computer, it connects itself to the Internet downloading several malwares, such as VIRUSES, WORMS, DIALERS etc.
DIALERS create a connection to premium-rate numbers, such as 800 and so on.What's the results? Huge bills!
SPYWARE spies on you while you surf the web, redirecting you to porn websites, changing your homepage and so on.