I NEED HELP! My computer has a horrible virus! HELP?

O.K. so I was surfing the web and my browser all the sudden closes. I can barley access the internet and my anti-virus program is no longer install and this virus installed some other anti virus call something like "unregistered anti virus 2011" or something, my anti virus WAS avg free. I cannot download anything and keep having these pop-ups "your computer is infected!" I open control alt delete and end it, that is also how I open the internet. I'm not sure what kind of virus it is and because my anti virus thing was uninstalled, I cant scan it or remove it and not mention download anything. PLEASE HELP WHILE I STILL HAVE INTERNET!