Scareware: Pop ups telling you that your computer has hundreds of virus problems , but all will be fine if you use your credit card to download their Software to clean up your PC.
Scareware will try and sell you a solution to a problem and it will sound very convincing.
Exit your web browser and run you own antivirus software, (assuming you have some, some free links).
Don't allow the software to download.
Don't pay by credit card or anyother means.
For the Techie's
If you see one of these popups showing you the drives that have been scanned, does it actually match the drive you have, probably not, what you see is a bit of flash graphics showing you the most likely drive combination. C:,D: and A:.