Microsoft's commitment to make its Windows operating system more secure appears to be paying off.

In its fifth semiannual Microsoft Security Intelligence Report, the company finds that vulnerability disclosures in Microsoft software during the first half of 2008 "continued a multi-period downward trend, both in terms of all disclosures and relative to total industry disclosures."

Microsoft Windows Vista stands out in this regard. Among browser-based attacks on computers running Windows XP, Microsoft vulnerabilities represented 42% of the total, with the remainder targeting third-party vulnerabilities. But just 6% of browser-based attacks targeted Microsoft software on Windows Vista machines.

"Because the operating system now has fewer opportunities for vulnerabilities, researchers and malware authors are looking elsewhere," said Jimmy Kuo, principal architect with Microsoft Malware Protection Center. "Vista is a lot harder to attack."

Based on Microsoft's CCM metric -- computers cleaned per thousand scanned by Microsoft's Malicious Software Removal Tool (MSRT), Vista has a significantly lower infection rate than XP.

It's not just Microsoft making progress on security. Microsoft's report finds that the number of vulnerability disclosures across the industry declined 4% from the second half of 2007 and 19% from the first half of 2007.

In keeping with a trend that other security researchers have observed, attacks are moving "up the stack," from operating systems to applications. More than 90% of the vulnerabilities disclosed in the first half of 2008 affected applications, Microsoft says.

Nonetheless, comparing the first half of this year to the last half of last year, the amount of malware and unwanted software rose by more than 43%. Some 30% of malware removed by Microsoft security products during the first half of 2008 consisted of trojan downloaders and droppers, continuing a proliferation of trojan malware over the past few years.

While the number of vulnerabilities may be declining, the amount of malware attempting to exploit those vulnerabilities is rising. In a recent meeting, Kevin Haley, director of product management for Symantec Security Response, said that his company had identified over 1 million unique malware signatures so far this year. That's more malware signatures in ten months than the 800,000 Symantec has identified in total over the past two decades, he said.

To explain this surge in malware, Kuo suggested, "There are more bad guys looking for potential monetary gain."