Hi Lauranne,
It sounds very much like you've got a malware/spyware infection on your computer. This is software that gets downloaded and installed by certain unscrupulous websites - one of the symptoms is to pop up these kinds of messages telling that you have got a virus or some other form of unwanted software, and giving you the option to get rid of it by installing the highly suspect applications that they are trying to distribute.
Get yourself a reputable spyware scanning application, and give your computer a good scan. Two of the best are
Spybot Search & Destroy (
The home of Spybot-S&D!) and
AdAware (
http://www.lavasoftusa.com/products/..._personal.php). Both are free to download and are fairly easy to use.
I've had a fair bit of experience with ridding computers of spyware, so
let us know how you get on or if you're still having difficulties.