Maybe infected

Hello,
Im venezuelan, and i have a problem on my computer, three week ago my computer doesn't want open the programes like antivirus, clean cleaner others..., i have antivirus avg and i can not open it , 'cause my computer doesn't let me, i think that is a virus, and i tried download others antivirus, but the virus doesn't let me open those pages, and i really need your help, you know giving me information about pages which i can download antivirus, some people has told me that i have to install windows xp again, and i have too much information, documents, aplications, in my computer and i don't wanna lose it... thanks.

por favor cualquier informacion sera de gran ayuda , no quiero perder la informacion que tengo guardada en mi pc, gracias de antemano. fray E