IS blocked really "blocked"?

Yesterday, my Norton flashed a popup stating it "blocked" an attack from "Web Attack: Malicious SWF Download 2". Then today, another popup stating it blocked an attack from "Web Attack: Exploit Toolkit Website 33". Both report "No action required". Now here's the thing... i believe these are virus' that are current ON my computer already and are attacking from within. My Norton didn't block them from infecting me, but blocking them from doing 'damage' now that they're here. Am i correct? (The second one even reported that the attacking PC is my own PC username).
Two days in a row, and so suddenly, can't be a coincidence. What do i do about this? i need an effective solution please, not just advice or ideas.