According to Volume 14 of the Microsoft Security Intelligence Report, fully 25 percent of all computers don't have up-to-date anti-virus software installed. Without that protection in place, the report states, those computers are 5.5 times more likely to be infected with malware.

"People intuitively understand the importance of locking their front doors to prevent their homes from being broken into," Microsoft Director of Trustworthy Computing Tim Rains said in a statement. "Computer security is no different. Surfing the Internet without up-to-date antivirus is like leaving your front door open to criminals."

According to the report, 25 percent of computers remain unprotected for three key reasons: a trial period has expired or the software is out of date, the anti-virus software was disabled by malware, or the user simply doesn't realize the importance of anti-virus software, so they don't install it.

"Regardless of whether you use a free or paid-for solution, the importance of anti-virus cannot be overstated," Rains added. "By taking the proper measures to protect your computer, including the most basic step of installing antivirus, you can dramatically reduce your risk of becoming a victim."