What do you think happens when you connect your computer to the Internet? In less than an hour, it may not be yours anymore. While you're Googling your name and checking e-mail, a hacker, perhaps in Eastern Europe-let's call him Ivan-quietly takes over your machine. There are a dozen ways Ivan could do it, but he probably found you with a program he didn't get at Best Buy called a port scanner, which roams the Internet like a clumsy cat burglar, trying every doorknob until it locates one left unlocked. Then he makes a connection to your computer-sort of like starting a chat session, only invisible to you-and uses it to deliver a "backdoor," a small piece of code that lets him take control of your PC whenever he wants. You won't know it, but you've just become part of a "botnet," a small army of computers that Ivan will rent to international organized-crime rings, which will use it to spew spam, steal identities, or hold corporate Web sites hostage (not to mention slow down your PC).