Sometimes referred to as "scareware", this program often appears in a fake window reporting to you that you already have hundreds (or thousands) of viruses on your PC. The unsuspecting user is then encouraged to purchase an antivirus product designed specifically to address the problems.
At this point, we have at least two facts: anything you click on this rogue window will be a mistake, and the attempted purchase of such is actually the worst decision that could be made because then the perpetrator will have all of your credit card data on the card you use for this transaction.