The true measure of a security tool's usefulness is often whether it runs on a wide range of systems without interfering with other software, Hansen said.

"This may work fine when you have it in the lab, but it's another thing when you try to deploy something like this on peoples' computers," Hansen said. "In fact, I could see something like this easily breaking the functionality of some leigitimate software applications."

Indeed, legitimate programs designed to automatically download security updates could encounter problems with a program like BLADE, said Eric Howes, director of research services at Sunbelt Software, a security company based in Clearwater, FL. "I would be especially concerned about potential false positives on other applications that perform background [software] updates or download stuff in the background."

BLADE certainly can't stop all Web-based malicious software, either, Porras admits. It cannot, for example, stop social engineering attacks, in which a user is tricked or bullied into installing a malicious program. The "Koobface" worm, for example, spreads on social networking sites such as Facebook and prompts recipients to download a video player plug-in in order to view a picture or movie supposedly sent by a friend. BLADE would do nothing to block such attacks because they ultimately prompt the user to install the bogus plug-in, which is in fact malicious software that gives attackers complete control over the victim's PC.

BLADE also is useless against threats that reside completely inside of a computer's temporary memory space, as the tool is designed to block malware that tries to write to the computer's hard drive. While most malware is written to the hard drive, there are some advanced threats that live only in memory.