The security company registered the biggest increase in malware in four years during the first quarter of this year, bringing the total number of samples to 83 million. Fake antivirus programs declined in popularity, but software with faked security signatures, rootkits and password-stealing Trojans rose.

McAfee counted about 200,000 new examples of password-stealing Trojan horses. A Trojan horses is a stand-alone program that represents itself as some form of legitimate software.

Rootkits are stealth programs designed to enable privileged access to the user's computer. The report calls rootkits "one of the nastiest classifications of malware." The Koutodoor rootkit spread fastest last quarter.

Software is "signed" by the vendor to tell users it's safe to install. A user is more likely to trust Microsoft or McAfee, for example, than an unknown vendor. Scammers capitalize on that trust when they forge the digital signature of a trusted provider in order to boost the chances of having their malware successfully installed on the user's computer.

Security researchers that forged security signatures would take off after the success of the proliferation of the malware programs that used that deception.