Anti-virus firm Sophos released its list of the top ten of the world's most common viruses yesterday, with the Klez worm holding on to poll position for the fifth month running.

The Klez-H worm, first reported back in October 01, has infected thousands of computers around the world and makes up around 18 percent of all virus complaints made to Sophos.

"While Klez tops the virus chart again this month, more recent discoveries, Frethem and Yaha, are close on its heels," said Graham Cluley, senior technology consultant at Sophos.

The Klez's H variant is not that deadly as, while it does cause havoc as it spreads itself from the infected PC via email, it doesn't delete any code. But the E and G strains of the bug contain time activators, which means they will reactivate on certain days, and start the whole disruptive process again.

To prevent your PC from infection, it is important to install an anti-virus package, but even once that is done users should remain vigilant to keep computers secure.

"People do not pay enough attention to security," said Shawn Woolfe, director of communications at anti-virus firm

"Viruses, hackers and other malicious code already costs businesses and consumers billions of dollars each year. People must learn to be far more vigilant," added Woolfe.

Top ten Viruses for July

1. W32/Klez worm (H variant)
2. W32/Frethem
3. W32 Yaha-E
4.W32/ Badtrans-B
5. W32/Elkern-C
7. W32/Hybris-B
8. W32/Klez-E
10. W32/Magistr-A