I have the trojan virus plus more all i can get on is firefix everything else dont work! PLEASE HELP!?

Ok so i downloaded a file yesterday and after it finishd all of a sudden my computer went crazy! saying something was trying to get into my computer or something and it lockd up my start menu and the computer it constantly turns off and on cept when im on fire fox i got to fire fix because i used the windows run on and it workd so i need to knwo is there a way to fix my computer! to get rid of these viruses FREE and quick! Where the time is supposed to be it just says virus alert! PLEase if any one knows what im talking about or how to fix this please let me know! with out having to pay any money!