Only one new piece of malware was prevalent enough to make it into the 10 most-active viruses last month as users continued to ignore updates, according to anti-virus vendor Sophos.

The Zafi-D virus, which was first discovered late last year, remains at the top of Sophos' top 10 virus list, accounting for 46.6 percent of all infections in April 2005. Mytob-Z was the only virus discovered in April to make the top 10 and was responsible for 1.3 percent of all infections.

Carole Theriault, security consultant at Sophos, said that variants of Zafi -- Zafi-B is in third position, accounting for 4.5 percent of all infections -- have succeeded because they are multi-lingual.

"Old viruses were still taking advantage of poorly protected computers in April... The Zafi family of viruses accounts for over 50 percent of all the viruses reported... Perhaps the success of these worms lies in their ability to spread in multiple languages, catching out unwary users all over the world," said Theriault.

According to Theriault, although the Mytob virus has infected a relatively small number of people, it is "nasty".

"Mytob-Z only accounts for a small percentage of the top ten reports... First sighted in mid-April, Mytob-Z is a nasty piece of work -- not only does it spread ferociously, but it plants a backdoor Trojan horse which can be used by remote hackers to gain access and control over a victim's computer," said Theriault.

Spyware continues to be a growing threat to PC users, according to antivirus firm Panda Software, which published its malware round up for April 2005 on Friday. According to Luis Corrons, director of PandaLabs, although users are more aware of the threat from viruses and worms, they are not yet savvy to the risks posed by spyware.

"Very often the full magnitude of the threat of spyware is not appreciated. The real problem with this type of malware is that in addition to the damage it can cause, which is significant, it steals information, examines users Internet habits and can be an entry point for other types of annoying malware," said Corrons.