For future reference:
One reason so many machines are corrupted is the use of Internet Explorer with 'active scripting' enabled. [This is that setting: IE> Tools> Internet Options> Advanced> Security: Allow active content to run files on my computer]
This setting allows a webpage to install software to your system, which may or may not be malicious; you have no way of knowing. In effect, it's an "Open Door" policy.
Another serious oversight is surfing while logged in on an 'Administrative' account.
One of the best ways to avoid malware being installed from the internet is to use Firefox with 'NoScript' add-on as your primary browser, and fall back on IE as a last resort, to view or interact with a sub-standard webpage. Switch back to FF when finished.
Firefox download: http://www.mozilla.com/en-US/firefox/
NoScript add-on: https://addons.mozilla.org/en-US/firefox/search?q=Noscript&cat=all