Well, last resort should really really be the last resort. If you can find something to do short re-formatting, go for it. One scenario where it would probably be necessary is what happened to me this past week. I went to go look for lyrics to a song and this malicious pop up ad installed a crapload of adware on my computer. One of them apparently contained a virus because my ISP dropped me from the network. I ran all sorts of programs to get rid of it and I got rid of most of it, except one very stubborn one. Right before it would get deleted it would make a duplicate of itself that wasn't targetted so that my computer would stay infected. I looked at my task manager and I didn't see any programs that looked suspicous running and there were no other files in my systems folders that I could find that were unfamiliar, so I figured it must be really well hidden and I was going on a wild goose chase with that one file since it kept copying itself, and I couldn't find its source, so that I figured warranted a re-format. XD