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:
Meet Your Favorite Search Engine Watch Contributors
Many of SEW's leading expert contributors will be at ClickZ Live, the new online and digital marketing event kicking off in New York (March 31-April 3). Hear from the likes of: Thom Craver, Josh Braaten, Lisa Barone, Simon Heseltine, Josh McCoy, Lisa Raehsler, Greg Jarboe, Dan Cristo, Joseph Kerschbaum, John Gagnon, Eric Enge and more!