First was the blaster worm. Of course, paranoid 'super smart' oldest bro had to 'take control of the situation', nuking all the files without giving anyone a chance to recover a thing, and constantly uninstalling things that people would install after he reformatted.
Later something that caused research ads.
Then I don't recall anything between then and when I got a computer to myself.
As for things that happened to computers I owned, just rogue antiviruses, and one keylogger.
One rogue was accidental. Don't recall how it came to be.
One rogue was intentional. I was testing to see how long I could use ie6 before getting a virus just by visiting webpages. It came from some off-site no one's ever heard of that had a pokemon typechart on it.
And the keylogger was curiosity. I downloaded a file that I knew clearly was an exe instead of the intended format. I disconnected my network before running it and sure enough, it was making a .txt file, containing my keypresses per application (by window text), being sent to some e-mail address that didn't exist anymore. So thankfully if anyone got this keylogger, nothing would actually happen. This was tested in a VM for safety.
