I think my pc has a virus

Recently my pc won't run my programms won't allow me to start taskmanager it only shows my background when i hit ctrl+alt+delete i can open task manager but i cant see it its like theres a screen onver my screen i can hit rigth click to open cant see start menu whit i hit sign out i can see my prgramms for a second and when i sign in i get a whit / black screen and after that my back ground

i run on windows vista
windows 7
lg montor
acer desktop

i dont know alot about pc's