Although I agree with you very much, I'll mention that you can format the hard drive as a good guy, too. It's not something necessary in all cases, but there are some where it is.
Consider a computer so flooded in Bad Stuff that all anti-virus software sites are blocked, and visiting any web site other than a popular search engine takes you to a spammer's web site made to look like a legitimate web site. (Example: You go to Yahoo, put in a search for hsn.com, then click the first link, for HSN's web site, and you instead end up at a car sales web site which isn't really a legit car cales web site, but it -looks- legit. Note that the user of this computer is an older user, and has very little computer knowledge, although there's been a lot of self-taught progress over time.)
Now you get to this computer, and you can't download any anti-virus software, or AdAware, Spybot, etc. No problem, they're either on a thumb drive you always carry around, or you can download them via your laptop. Only, when you go to install them, they're being blocked from installing by something.
At this point, I see two options: 1) Spend more time than it's worth trying to clean the computer, or 2) Move all the user's files to an external hard drive, format the hard drive, reinstall Windows (which is easy when there's a Windows XP recovery partition on the hard drive), then re-create the users and return their files to where they should be.
While "repairing" computer software issues isn't "what I do" commonly, it's something I do from time to time. I definitely undervalue what I do, but I let it be known what it would cost to take the computer in to a repair shop, etc., and that typically makes the computer's owner realize that they're saving a -lot- of money with my services, so they're willing to pay more than they might have otherwise. (And it's legit, as they -are- saving a lot.) I like to give them a price range, such as "$50 to $150, depending on what it's worth to you to be able to do such-and-such with your computer again", so they realize right away that $10 isn't going to cut it.
That said about undervaluing my work, even though it's infrequent that I do this, I've learned quickly that my time is worth too much to spend it trying to clean a really infected system. It's less time and effort for me to to relocate the files and give the system a fresh install. Sure, some settings are lost in the process, and programs need to be reinstalled, but that's less painful than cleaning the infection, and the computer'll run better for a while (which seems to be true of any fresh install, be it Windows XP, a freshly installed new version of Ubuntu, or even wiping out ~/.kde/ [sans important files]).
If I were doing this sort of thing more often, I'd carry with me 1) an external hard drive to copy files to, 2) an external DVD drive (some people have $199 computers with -slow- drives), 3) a Linux LiveDVD (easiest way to copy files over), and 4) I'd write a script to automate copying files to the external hard drive, then back to the computer's hard drive after re-installing Windows.
