As with similar filters found on browsers such as Mozilla Firefox, the first thing that Smartscreen does for IE8 users is check each site visited against a whitelist of known good sites, further assessing any new site in case it is suspicious.
Secondly, and more importantly, if the user chooses to visit a suspicious site, IE8 will still automatically stop files from that site from being downloaded by the unwary.
This is the element of blocking referred to in the 'one billion' milestone. The feature first appeared in IE8 when it was released in March 2009.
"We have got better and better at blocking malware through the SmartScreen Filter because we have continued to invest in our back end service," said Microsoft's senior product manager, James Pratt, in a blog.
For comparison, by August 2009, the filter had blocked only 70 million downloads, the company said. Having reached the one billion mark almost a year later implies that Smartscreen is now blocking roughly that number each month, albeit with a larger share of the browser user base.
Anti-malware filters - or anti-phishing filters as they used to be called - are a relative backwater in security because all browsers have some version of the technology. Comparisons between browsers are treacherous but IE8's file blocking could possibly be giving it an edge in this particular aspect of security.