When I was 15 my mother decided that Bonzai Buddy would be a cool thing to try.
When I was 19 a bug in IE allowed the CWS trojan (or spyware, if you prefer) to infect my system without my consent.
<vader> NOOOOOOOOOOOOOOOOOOOO! </vader>
Once, my brother's PC was acting funny, so I reformatted his HD (with his permission and after backing up his important documents, FYI), and the message "Yoyo" appeared in funny looking ASCII characters (I believe that's a boot sector virus). The problem seems to have gone away with the reinstallation of Windows, but I made the mistake of neglecting to reinstall his antivirus, and he has three children (so I volenteered to clean up his system, and did it ever have a lot of Badware™ on it).
Most of the worsed things that have happened to my computer arose from my own stupidity, but I am pretty much immune to that, now, even without a ton of scanners I am pretty weary of what I install simply from past experience.
The worsed mistake I have made with my computer is ESD, (I fried it with static). I was careful, but you can never be too careful.
My level 93 debater is no match for your level 1 moderator