Hah, you got ransomewared. Yeah, get rid of that thing ASAP. If it's simple enough you should be able to easily remove it with Microsoft Security Essentials, the free version of avira (I recommend it ), or the free version of bitdefender.
The others are right, but let me be a little more precise. This stuff is known as Scareware. It finds a security hole in your PC, installs itself, and then pops up saying it has found viruses. (Some other variants that I've seen involve fairly simple webpage redirects that show you a picture that imitates the XP My Computer screen and names your drives "173 viruses", etc. Then it shows an information popup alerting you that the scan has found all these viruses, go to this website to clean them up.) It tells you to buy a program to remove it. If you take the bait, the crooks get your money, the popups go away (at least for a time), and the "antivirus" you just bought is sometimes malicious software itself, e.g., a keylogger to steal your passwords. If you have the webpage variant, all you have to do is close that tab. If it's the kind that actually installs itself on your PC, then you will need an antivirus or antispyware program to remove it.