A "threats report" released late Monday by McAfee showed that the number of new pieces of malicious code, or "malware," targeting Windows machines in the first three months of this year was the highest in four years.

There was also a rise in malware aimed at mobile gadgets running on Google-backed Android software and at Macintosh computers based on Apple operating systems, according to the report.

"In the first quarter of 2012, we have already detected eight million new malware samples, showing that malware authors are continuing their unrelenting development," said McAfee Labs senior vice president Vincent Weafer.

"The same skills and techniques that were sharpened on the PC platform are increasingly being extended to other platforms, such as mobile and Mac."

For a long time cyber criminals concentrated their attention on getting into Windows-powered personal computers (PCs) because the popularity of the operating system meant a wealth of potential victims.

But as Apple and Android devices have caught on around the world, they have emerged as a new target.

"As more homes and businesses use these platforms, the attacks will spread," Weafer said.

While Mac malware has been steadily proliferating, it is still a small fraction of what has been developed for PCs, according to McAfee.

The amount of spam in the quarter dropped to slightly more than a trillion trash messages monthly, with significant decreases in Brazil, Russia, and Indonesia and increases in China, Britain, Germany, Poland, and Spain.

The number of networks of virus-infected computers, referred to as "botnets," rose to nearly five million, with Colombia, Japan, Poland, Spain and the United States seeing the biggest increases.

Hackers use ploys such as booby-trapped emails or links to secretly infect machines with malware that can let them not only steal data or track keystrokes but use the infected machines to launch further attacks.

The United States was the primary address for botnets and cyber attacks in the quarter, McAfee reported.

Cyber crooks sell malware software kits and rent out use of botnets, according to Internet security specialists and law enforcement.