virus, can't open hijack this

I have a virus that won't let me ope hijack this, AVG, or ccleaner and continues tgive me warnings that my computer is infected. ANy advise ??

Please visit this webpage for instructions for downloading and running ComboFix:

http://www.bleepingcomputer.com/combofix/how-to-use-combofix

Post the log from ComboFix when you've accomplished that.