Most PC viruses are pretty obvious, especially the current breed of instant killers that Mac Defender is modeled after, the virus warning trojan instant nuke has become very popular with the script-kiddies.
Oh yeah, I got one of those, where when you click to close the pop up it installs it, but I also got another one that must have been resident for a bit before I knew it was there. It turned off my updates and did some damage. I’m pretty unsaavy, so I can’t explain, but it destroyed my computer. Fortunately it was an older computer and I replaced it with a Mac. I was able to retrieve my data before the thing gave up the ghost.