Users are at risk from new variants of popular viruses which can evade some antivirus protection.
The first documented example is Pretty Park, which first appeared last June, but has since been decompressed and redistributed. Even though users may be protected against the original version, they will need to upgrade their antivirus software to detect the decompressed version.
Pretty Park is a so-called internet worm that appears as an icon of a character from the cartoon series Southpark and attached is the file Pretty park.exe. When opened, the program mails itself out to all the email addresses in a user's Outlook Express database.
Eric Chien, head of research at antivirus software vendor Symantec, said: "The latest version of Pretty park is different to the original, and unless companies have downloaded the latest antivirus updates they won't be protected. This virus writer has taken the old version, decompressed it and redistributed it.
"Updates to antivirus software should be done at least every week. All the major antivirus vendors have updates every week, plus extra special definition sets. It doesn't hurt to check antivirus websites every day - this can be automated."
Chien said that seven out of the top 10 viruses in Europe in February used networks to spread themselves around. Pretty park was the third most common virus, after wscript/kakworm and happy99. The number of viruses using networks will increase to 10 out of 10 in the next few months, said Chien.
David Emm, sales development manager at Network Associates, agreed: "We have seen an increase in these types of virus over the last few weeks. There is an increasing need to keep on top of updates. A lot of our customers were only doing these quarterly, now many of them have changed to weekly."
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