The firms are supporting the emerging standard known as domain keys, which block fake e-mails by validating the sender with a digital signature.

Spammers hide their identity by using a false, or spoofed, address in the millions of messages they send out.

The technology, called the DomainKeys Identified Mail (DKIM), will be available to millions of Yahoo Mail users worldwide in the coming weeks.

It is a big step forward for consumers in defence against the bad guys," John Kremer, vice president of Yahoo Mail, told Reuters news agency.

Several ways. First, it can allow receiving companies to drop or quarantine unsigned email that comes from domains that are known to always sign their emails with DomainKeys, thus impacting spam and phishing attacks. Second, the ability to verify sender domain will allow email service providers to begin to build reputation databases that can be shared with the community and also applied to spam policy. For example, one ISP could share their "spam vs. legit email ratio" for the domain www.example.com with other ISPs that may not yet have built up information about the credibility and "spamminess" of email coming from www.example.com. Last, by eliminating forged From: addresses, we can bring server-level traceability back to email (not user-level - we believe that should be a policy of the provider and the choice of the user). Spammers don't want to be traced, so they will be forced to only spam companies that aren't using verification solutions.

Why do some of my search results say "This site may harm your computer?"

This warning message appears with search results that we've identified as sites that may install malicious software on your computer. We want our users to feel safer when they search the web, and we're continuously working to identify such dangerous sites and increase protection for our users.

Malicious software is often installed without your knowledge or permission when you visit these sites. Some examples of malicious software include programs that delete data on your computer, steal personal information such as passwords and credit card numbers, or alter your search results. For more information on these types of sites, please visit http://www.stopbadware.org/home/help

Even if you click on the title, you won't be able to visit the site: Google explains you once again that the site could be dangerous and recommends you to visit another search results or to change your query. The only way to visit the site is to copy the URL and paste it in the address bar, which is not an obvious or a trivial task for an ordinary computer user.

My Web interface will identify specific malware without the Google API. It directly searches Google using fingerprints from executables that we already have," he said.

Moore's project uses code strings, or fingerprints in malware samples, then runs a search on Google for those characteristics.

The search engine has been programmed with about 300 malware signatures and Moore said he plans to add another 6,000 signatures in a future bug fix update.

WARNING! The following links lead to executable programs that can harm your computer. Please be careful when accessing these files and make sure that you only run them in a restricted environment. The signature technique used by this search engine may result in non-malicious files being detected as malware, so be sure to use an updated virus scanner prior to filing a complaint with any web site.

Offensive rootkits, which are used hide malware programs and maintain an undetectable presence on an infected machine, have become the weapon of choice for virus and spyware writers and, because they often use kernel hooks to avoid detection, Danseglio said IT administrators may never know if all traces of a rootkit have been successfully removed.

He cited a recent instance where an unnamed branch of the U.S. government struggled with malware infestations on more than 2,000 client machines. "In that case, it was so severe that trying to recover was meaningless. They did not have an automated process to wipe and rebuild the systems, so it became a burden. They had to design a process real fast," Danseglio added.

Danseglio, who delivered two separate presentations at the conference-one on threats and countermeasures to defend against malware infestations in Windows, and the other on the frightening world on Windows rootkits-said anti-virus software is getting better at detecting and removing the latest threats, but for some sophisticated forms of malware, he conceded that the cleanup process is "just way too hard."

"We've seen the self-healing malware that actually detects that you're trying to get rid of it. You remove it, and the next time you look in that directory, it's sitting there. It can simply reinstall itself," he said.