A worm that is spread via USB flash drives has been named the biggest security threat to PC users by McAfee.

According to the security vendor's Threats Report: First Quarter 2010, an AutoRun related infection was also the word's third biggest PC threat, while the rest of top five biggest PC threats were made up of password-stealing Trojans.

The report revealed that Spam rates have maintained steady.

However, there has been an increase in diploma spam, or spam that offers forged qualifications, in China, South Korea and Vietnam.

McAfee also said malware and spam in Thailand, Romania, the Philippines, India, Indonesia, Colombia, Chile and Brazil had surged.

The security vendor said this was down to the significant growth of web use in these countries coupled with a lack of security awareness.

"Our latest threat report verifies that trends in malware and spam continue to grow at our predicted rates," said Mike Gallagher, senior vice president and chief technology officer of Global Threat Intelligence for McAfee.

"Previously emerging trends, such as AutoRun malware, are now at the forefront."

The security vendor said attackers continue to use major news stories to ensure web users are directed to malicious web links, while the US continues to host 98 percent of these poisoned links.