If you see web links for scan4home.com do not follow them. If you do follow the link then you will be presented with what looks like a virus scan operation followed by warnings and advice to make changes to your machine. This presentation is a remarkable forgery of a Microsoft Windows virus scanning tool. If you follow the advice I suspect you will end up infecting your machine. Don't do what it asks.
Scan4home.com make it nearly impossible to escape the web browser. The only way I know how to do it is to end the web browsing process. Ie, kill it. To do this you need to use the "Task Manager" tool that comes up with pressing Ctr-Alt-Del. In the list of processes presented find all Internet Explorer, Firefox, and/or Safari processes and end them. If you are unsure how to do this just restart your machine and stretch out while the machine is restarting.