A while ago I installed Ubuntu Hoary on the home PC of my girlfriend's father, because the existing Windows installation was broken (to the point of not starting Windows) by vast amounts of ridiculous worms, spyware, adware, and viruses. Nevermind that it would be unsafe for online banking and email even if it half worked. I could have re-installed Windows but I couldn't guarantee that it wouldn't be infested quickly, and I've never met anyone capable of doing that either. He's had people reinstall it a couple of times already.
I've discovered the same awful situation on several people's home computers running Windows. These people live with the instability, confusion, and fear. There's even an “anti-virus” program that's installed by default on many computers here. It doesn't do much more than repeatedly warn you (via the notification area) that “your computer is in danger” (a rough translation of the German). It has a GUI to make your computer not in danger but it's insanely complex, so people's computers will of course remain in danger so they are encouraged to feel it's their fault.
I bought a new laptop last week. After a tedious post-install wizard, the “your computer is in danger” message is the first you see. Then it restarts twice, showing you the message again each time. It should say “Your computer is in danger. Get Linux.”, or “Get Linux with easy-to-use GNOME. Don't be afraid any more.”.
The Ubuntu PC works fine for browsing, email, document editing. He just uses it and doesn't seem disturbed by the change. It's still working.