Windows Live Messenger Wave 4 tie in Internet Explorer's SmartScreen Filter technology. Basically, links you receive will be transformed in such a way that upon clicking them you'll be pushed through a redirector controlled by Microsoft before reaching end result… if deemed safe. To be a little more specific, you're sent to the ominous appearing http://rdir.us with some undocumented parameters tacked onto the end. A full URL may appear as such: http://rdir.us/?l=http%3a%2f%2fyoutube.com&h=unknown_base64ed_value&p=number&u=sixteen_hex_digits.
If it's safe to proceed, this redirect shuffle takes only a second or two to complete. If things look a little bit fishy, Microsoft will throw up an interstitial page (see pic):