Microsoft's Live Search is now flagging sites which they deem questionable in terms of malware. Users can still visit the site, but you'll have to click a special link in the warning box, as the main link has been disabled.
If you find that your site has been flagged (perhaps a third party hacked your site), you can use Live Search's webmaster tools to get back on track.
Here's a screenshot of what a warning would look like:
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