antivirus xp 2010

I nearly gave them money! How can I remove this virus?

 

I am using windows xp on a sony vio machine. I have norton antivirus installed but cant get rid of "antivirus xp 2010" it keeps saying privacy threat, system hijack, stealth intrusion... etc and scanning my computer telling meIi have infections. I can't open firefox and it keeps asking me for money.

 

Thanks -Paul