Criminals changed tactics in the last six months of 2007, dropping malicious e-mail in favor of Web-based attacks, according to data reported to Microsoft by Windows users.

The company saw the number of Trojan downloader programs it removed from Windows machines jump by 300 percent, according to Jimmy Kuo, principal architect with Microsoft's Malware Protection Center. These programs masquerade as legitimate pieces of software, but once installed they then download malicious software such as spyware or adware onto the victim's computer. They are typically installed via the Web.

The shift to the Web has been forced onto criminals, as system administrators have become better at blocking executable files from being sent via e-mail. So instead of sending their malicious software directly via e-mail, the bad guys are now being forced to send out spam messages that trick victims into visiting the malicious Web sites. "Executables are often being stripped completely regardless of what they are," Kuo said.

Many companies compile data on Web attack trends, but Microsoft's is the most comprehensive -- based on data from the approximately 450 million computers that run the Microsoft Malicious Software Removal Tool that ships with Windows.

Kuo said that there are still a lot of infected Windows boxes out there, although there are fewer than some have reported. On average, Microsoft removed malware from one out of every 123 computers it inspected each month during the period. In the U.S., that number was 1 in every 112. Japan was the least-infected country, with malware found on just one in 685 machines.

Microsoft published its findings Monday in its Microsoft Security Intelligence Report, Volume 4.

Other data from Microsoft's report:

* The total number of malware items removed by Microsoft's tool was up 55 percent from the first six months of 2007.

* Adware is still the most common form of unwanted software, and was up 66 percent in the second half of the year to 34.3 million detections. The top piece of adware for the period was Win32/Hotbar, which installs an Internet Explorer toolbar that spews pop-up ads onto the PC.

* Between 75 and 80 percent of phishing pages tracked by the Microsoft Phishing Filter were in English, and phishing is now moving from e-mail onto social networks.

* Rogue security software is on the rise. The most widely spotted of these bogus or malicious programs that pretend to protect PCs was Win32/Winfixer. It popped up five times as frequently as its nearest rival.