How can you tell if your computer has been taken over by a "Bot Net" ?

[insider]

The simple explaination is that somehow (via website/e-mail etc) a central host infects thousands or millions of PCs. These PC's are "blindly" part of an attack on systems. It is setup to either bring down servers/ prevent access or sometimes mass attempt phished (stolen) logins on popular banking websites.

I'm not sure that any have targeted the mac (yet) so the whole windows startup screen is pretty valid.