Are you a Windows user? If so, you are hopefully aware of the insecurity of your operating system. And since you are aware, I guess you protect your computer against viruses, worms, trojans and other nasty stuff with the appropriate software. Expensive software by the way which is only needed on Microsoft Windows computers, but this is not the point here.
Do you know what Botnets are? In short, a Cracker tries to infect as many computers as possible with a trojan to take control of it. But you won't notice if you don't pay particular attention to what's going on on your computer. Can't happen to you, right? Yeah, sure; let me get the facts (collected by Rixstep):

• 11% of computers online contain botnet code; affected machines are exclusively running MS Windows
• 250,000 new Windows systems join a botnet every 24 hours
• 80+% of all spam is generated through botnets today

OK, let me get this straight: One in ten Microsoft Windows computers out there is part of a botnet. Beside the fact that the criminal controlling your Windows computer can read everything you have saved on your computer – including addresses, credit card numbers, stock brokerages, bank accounts, email accounts and so on – you are responsible for 80% of all spam emails generated today!!
Still using a Windows based computer? Your choice.

Update
As you can see in this PDF File, one year later, same situation. About every single spammail is sent from the friendly Windows-User from the neighbourhood...