Hackers are exploiting a new, unpatched vulnerability in Microsoft Word that could allow them to take control of a victim's computer, Symantec has warned.

The zero-day vulnerability is the fourth in Microsoft's widely-used Word 2000 software that has not yet been patched, Symantec said in its Security Response Weblog.

A zero-day vulnerability refers to a security hole for which exploits are already available when it was discovered. This latest one affects most versions of Windows running Word, Symantec's advisory said.

Danish security vendor Secunia ApS also reported the vulnerability, and rated it as "extremely critical", its highest-level warning.

The attack comes via an infected Word document, a method increasingly used by hackers for targeted attacks. If the document is opened, it installs a Trojan horse program, called Trojan.Mdropper.W, onto the computer. The Trojan also puts other files on a computer that enable a hacker to control it.

Microsoft could not be immediately reached for comment. The company released three sets of critical patches on on January 9, including ones for Outlook, PowerPoint and Windows, but not for Word.

Users can avoid trouble by not opening unexpected Word documents attached to email. Hackers often send out thousands of spam messages with harmful attachments, such as Trojan horse programs, hoping unsuspecting victims will open them.

Trojans often look harmless and can quietly install themselves on a computer with no visible signs. The use of Word to mount an attack can be particularly effective since the file format is so widely used.