Microsoft has advised people to run its Word application in "safe mode" to block a recent Trojan program, although security experts say there appears little cause for alarm.

"The good news is that it doesn't seem to be very widespread," said Graham Cluley, a senior technology consultant with UK antivirus company Sophos. "There have been very, very few reports."

Researchers at F-Secure Corp. and Trend Micro Corp. also said the number of reported incidents remained low on Wednesday. Trend Micro rates the Trojan as "low risk" because, while the potential for damage is high, the impact so far has been small, said David Sancho, a senior antivirus engineer.

The Trojan surfaced last Thursday and arrives buried in a Word file attached to an e-mail message. It secretly installs software on a user's PC that could be used to execute remote commands, download other malware or monitor keystrokes and gather passwords, among other mischief.

For the Trojan to do its work, however, users must first be tricked into opening the Word attachment. And the incidents reported so far suggest that hackers are still using the Trojan in a very targeted fashion rather than sending it in mass e-mail, said Erkki Mustonen, a security researcher at F-Secure.

The Finnish vendor received reports from a handful of European companies affected last week that were all in the same business area, Mustonen said. He declined to name the industry. The company received a few more reports this week but "it seems to be pretty calm," he said.

The number of hacker groups using the Trojan appears quite small at this point, Mustonen said. "It seems they have been written by expert people," he said.

He advised businesses to monitor any suspicious traffic coming from China in their firewall. The Trojan may not have originated there, but it appears at least to be talking to a host server in that country, he said.

Microsoft's Security Research Center is analyzing the vulnerability, which affects Microsoft Word XP and Word 2003. The company said it will release a patch with its next regular update, due June 13, or earlier if necessary.

In the meantime,Word's safe mode won't fix the vulnerability but will prevent the vulnerable code from being exploited, Microsoft said.

The first step is to disable the Outlook feature that uses Word for editing e-mails. The second involves creating a new desktop shortcut that adds "/safe" to the Word command line. Detailed instructions are in a Workaround section at Microsoft Technet.

"For the sake of security I'd recommend doing it, even though it's a bit difficult," Sancho of Trend Micro said.

In safe mode, Word ignores toolbar customisations, changes to preferences can't be saved and functions such as AutoCorrect and Smart tags are disabled.