What I do is to backup all of their important data, format the machine, install the operating system again, and install the latest version of Firefox on it. And boom! Gone are all the drive-by-downloads, the unwanted pop-up ads, and a nice side-effect is the increase in browsing performance.
And how do I ensure that they do not use Internet Explorer again (accidentally or otherwise)? The simplest way (and its quite effective too) is to remove its icon from, the start menu, the desktop, and the quick-launch tray. And I also make Firefox the default browser.
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Here's what someone has to say. Although it isn't about Firefox, the spirit nonetheless is the same...
"I'm actively contemplating installing (Ubuntu) for a few friends and family, because I'm sick of de-spywaring their Windows machines." --Stephen O'Grady, analyst, RedMonk
Read My Thoughts On Human Brain