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:
The Original Search Marketing Event is Back!
SES Denver (Oct 16) offers an intense day of learning all the critical aspects of search engine optimization (SEO) and paid search advertising (PPC). The mission of SES remains the same as it did from the start - to help you master being found on search engines. Early Bird rates available through Sept 12. Register today!