The caller tells you they will disinfect your computer at a very reasonable price. After (or, sometimes, before) you send the money, they tell you to click on some buttons and links, so they can disinfect your computer. However, if you do that, you give the scammer full access to your computer, and they will do nasty things, such as steal your card or bank data (if you had it on your computer), steal your contact list, and send friends phony emails. Needless to say, if you sent money, it is gone. If you gave THEM your card number, consider contacting your card issuer!