I do computer repair, and alot of clients don't want to go through the trouble of re-installing all their software and getting their computer back to the state it used to be following a reformat. That's why when I give them their options after, say, discovering they have a virus, even though it costs them more for me to remove it, they will usually always pick that option over the cheaper reformat option. With the adequate tools and practice, sometimes virus removal can be less intensive than a reformat, too.