HOW DO I GET RID OF ANTI VIRUS LIVE?????

okay, so i was surfing the net when suddenly one of those 'please scan now your computer is at risk at crashing' advertisements popped up.So I downloaded it and it started to mess up my computer and tell me Spy ware terminator was messed up, and then all these o pop ups came out of no where!!! I panicked and deleted the file....yet it still won't leave me alone and the pop up alerts are worse!!!! please tell me how the permanently rid of it!!!! TT _ TT