Virus and other things wrong with my computer?

ok for one thing there is a trojan virus on my computer i can't download any good anti viruses like norton or avg i tried housecall online scan it couldn't get rid of a worm and some malaware thing and to top it off sine today things like aim windows media player and ares weren't opening and it says windows can not open the specified device path or file. you may not have the appropriate permission to activate these files so i really need some help because my last resort is formatting my computer for which i fear because i don't think i have the windows xp installation disc.