The error message “Warning visiting this site may harm your computer” can be a true message that comes from Google, IE, Firefox or your security software. No matter what, you will find out it is more than easy to know if your browser has really been hijacked by some threat. If you ever see a software program being pitched or a recommendation to a product then the message is false and you should not trust it. For exact examples or to learn how to solve this check out this Warning Visiting This Site May Harm.
The same is true if you do a Google search and notice some extra text under the Google search box recommending out a certain security client. Another example would be a good website you do happen to visit on a daily basis. If all the sudden you see this message but know the site is a good one then you are a victim of a virus on your computer. You will find out that cleaning your system of those threats is not really easy. Another place to go to learn more about similar threats is this Spyware Removal site.
Your computer is indeed infected by some bad program if you do happen to read the same warnings on all webpages you are browsing. To solve this problem: try to reset your IE (or Firefox, or Chrome, …), sometimes it works. It will eventually clean the threat, but not for everyone. In any case: always perform a full system scan with your antivirus program.
If nothing has changed after that then your computer has a deeper infection with possibly more than just one virus. To help you to get rid of this threat (fake security app) : read a lot about it. Search for this program (just write it down somewhere) on one of the websites I did mentioned above. You will get some very detailed instructions about how to clean your system from the threat found.