You’d think that everyone using a computer today would run some kind of antimalware software. Apparently that’s not the case: almost one in four computers is surfing the web without proper security software.That’s according to Microsoft’s latest Security Intelligence Report. It’s an alarmingly high number, and not just because those machines are five and a half times as likely to pick up an infection as those that are properly protected.Security on the Internet is a group effort. All those inadequately defended systems pose a big security risk to the rest of us because they’re susceptible to today’s craftily-engineered trojans and drive-by downloads. It just takes one errant click and some botmaster has gained another new zombie to add to his or her army. More zombies equal more spam and more damaging DDoS attacks.Fake antivirus apps continue to be a major problem, but Microsoft notes that there have been a few notable successes. Last year, it helped clean up more than 3 million infestations of the Onescan fraudware that spread via poisoned PDF and Word files. Yes, people do really open those bogus shipping invoices that pop up in their inboxes.Another reason folks wind up unprotected is that they think their trial bloatware is still doing its job after the free period has run out. That’s just plain wrong, of course, because the updates stop coming and protection gets watered down like the drinks at a bad all-inclusive resort.The report isn’t all doom-and-gloom, however. There’s good news in Microsoft’s analysis, too. The number of infections spotted on computers in the U.S. dropped from over 12.4 million in the second quarter of 2012 to just under 9 million by the fourth. That’s still a mountain of infected systems, but it’s nice to see that progress is being made.