Watch out for Antivirus 2008/2009!
Have you recently received a pop-up message saying that your computer is infected with a virus and asks you to download a program (called Antivirus XP 2008 or 2009) in order to scan for and remove these viruses? If so, DO NOT click "Ok"! Whatever you do, DO NOT install this program on your computer. Read this article by Enigma Software Group to find out more about it.
Basically, this is a known trojan that infects your PC, causing random pop-ups claiming that your PC is infected with viruses and you need to purchase the full copy of Antivirus XP 2008/2009 in order to remove them. They then point you to a website to go to in order to purchase the program. Then, when you purchase it and enter a credit card number, you will start noticing unauthorized charges to your account.
This is scary stuff. Do not download. If you do by accident, DO NOT purchase the full version. Removal is very tricky and sometimes requires a full reformatting of the harddrive. So please be careful and good luck!