What is a "Zombie" PC

A Zombie PC is one that is no longer its own master, it has been taken over by a hacker (or cracker, to be precise). When your PC gets infected with viruses or some spyware, it allows for certain types of programs to be downloaded, sometimes in segments, (so you don't know it is downloading a large program, it just seems like your internet connection is running a bit slower than normal) and reassembled on your machine. Once it is fully in position, the program can execute, and tell the programmer it is now ready to use. The programmer can take control of your PC remotely (probably not like remote desktop where you see the mouse moving), but can send a load of code down to your PC and your PC then immediately acts upon that received set of commands. You may suddenly find that while you are word processing your PC is actually spewing out loads of spam emails. Or while you are doing your spreadsheets your PC is actually part of a worldwide attack on a website the other side of the world - or on the other side of the road - on someone who refused to pay "protection money".

People who have written these programs also sell the time on these zombie computers to others, so you might also find you have been helping the Russian Mafia. Certain individuals have been known to offer over 100,000 zombie computers for a concerted attack on an online betting website 3 days before Derby day because the online bookie wouldn't pay a few thousand to these fellows to "leave off". For a small consideration, of course. These distributed attacks are almost impossible to fight with technology.

Interestingly, recent news threats from the Mafia have been that "if you don't pay us these measly few thousand pound we will send out millions of paedophilic/pornographic Emails with your name on it. So if Cadbury or Coca Cola or Royal Bank of Scotland start sending out naughty Emails, be aware it probably really isn't them.
(It might even be you...

if you haven't got up to date Anti Virus software, that is)