First, do not panic
Second, the warning that pops up on your computer is malware designed to get you to purchase a program that will get rid of the supposed virus. What you have is adware on your computer and most likely NOT a virus.
Here is what to do:
1. Update and scan your hard drive with your resident Anti-virus. If you do not have one, there are several very good ones that are free.
2. Next, download one or two anti-adware/spyware applications. BC has a list of free applications here:http://www.bleepingcomputer.com/forums/t/34773/preparation-guide-for-use-before-using-malware-removal-tools-and-requesting-help/
Two of the best are Spybot and Ad-AwareSE. Install and update their definitions and scan your hard drive with both, letting them get rid of all the malware they find. These are good to have and run on a regular basis.
If, after doing this, you continue to see that pop-up, then submit a HJT log so our great team of volunteer experts can find the problem and help you get rid of it.
Carefully read and follow the instructions for submitting a log that is found here:http://www.bleepingcomputer.com/forums/t/34773/preparation-guide-for-use-before-using-malware-removal-tools-and-requesting-help/