Virus that hijacks security system?

I turned on my computer tonight and all of the sudden Windows XP security has taken over my computer. I cannot uninstall, I cannot delete or turn off, I cannot surf the web, because it claims to be an unsafe page and won't let me continue. It gives me an option to continue...but won't click the link. I can't do a thing and I don't know if my antivirus will even detect it since it claims to be a security enhancer. When it pops up, it tells me I am infected and need to scan...it starts automatically and (surprise!) finds 5 or 6 viruses...then takes me to a page to purchase a program to fix it. The only problem is that this page claims to be from Windows...yet has typos...such as "Widows" XP and "pervent" data loss. I am currently doing a more comprehensive antivirus scan but I don't know if it will help...any suggestions???