From Long Zheng:
Apparently Windows 8 will use SmartScreen as a security feature.
SmartScreen is practically a dynamic list of known “illegal, viral, fraudulent, or malicious” URLs that Microsoft manually or algorithmally identifies. It uses this list to compare the link or executes a user visits or downloads, warning if there’s a match.
It will proactively check the filesystem and attempt to stop and downloads that are foreign to it.
Source: I Started Something