Within the first week of having my first computer...
As a joke someone I knew sent me something which set off this looping gif filling my screen with the sound 'You are an idiot, you are an idiot, HA HA, HA HA!' on repeat. Restarting got rid of it.
Sometime after I must have done something as someone claiming to be from my ISP called me attempting to "fix what I'd done" and help clear up a virus they said I had. I don't recall if the subject of paying for anything ever came up, but I assume if that were the case and they realized I had no method to do so they finished up the call. Never heard from them ever again.
http://air-hammer.deviantart.com/