Users don't even need to open an attachment to be infected with the latest variant of the virulent Netsky virus.

Netsky-V, unlike its widespread siblings, spreads without using email attachments. That means users can get hit with the virus just by opening a tainted email.

''This makes me a little bit nervous because of the way it automatically infects machines,'' says Patrick Hinojosa, CTO at Panda Software U.S., an anti-virus and intrusion prevention company with U.S. headquarters in Glendale, Calif. ''It doesn't require foolish end users to spread. And anything that doesn't require user participation to work is bad news.''

Analysts note that a flood of worms and viruses have been plaguing corporate networks over the last few months. And those infections have occurred because users, despite being educated about the dangers of attachments, were either negligent enough to click on an attachment or they were conned into it. That leaves analysts worrying how bad a virus infection could get if a user doesn't have to screw up to make it work.

''This is a problem,'' says Hinojosa. ''Smart user, not a smart user -- it doesn't really matter. What matters here is if you have patched software. These are the viruses that can spread pretty fast... How successfully it spreads just depends on how well it was written.

''I'm just hoping this one wasn't written very well,'' adds Hinojosa.

So far, it's unclear how quickly this worm is spreading. Since it was just released into the wild, the numbers on it aren't really in it.

This new V variant has malicious XML code hidden in the message body of the email. When a user opens the email to read it, the code automatically seeks out a known object validation vulnerability in Microsoft Corp.'s Outlook and Internet Explorer software. The vulnerability allows the malicious code to be trusted, installed and executed on the local system.

Once the computer is infected, the malicious code will install a backdoor that listens to TCP ports 5556 and 5557. Netsky-V is designed to launch denial-of-service attacks on several Web sites between April 22 and April 28. The sites to be attacked include kazaa.com; emule.de; cracks.am; freemule.net, and keygen.us.

Ken Dunham, director of malicious code at iDefense, a security intelligence firm based in Reston, Va., says Microsoft released information about the vulnerability, along with a patch to correct it, early last October. If a system has been patched, Netsky-V will not be able to infect the computer.

But Hinojosa says there are millions of computers that have not been updated, so are vulnerable to the attack.

''There are millions of unpatched computers on the corporate side,'' says Hinojosa. ''Most U.S. workers work for companies with fewer than 25 employees. They don't have a system administrator. And that's not counting all the home users -- the millions of home users -- who haven't patched their systems in the last several months. How fast this spreads depends only on how well it was written.''