The security software in Microsoft's Windows Vista is "ineffective" in blocking malware and slow to update against viruses, according to a security software vendor. 

A test performed by Webroot Software found that Windows Defender, the security software included with Vista, failed to catch 84 per cent of spyware and malware. The test included 15 of the most common malware programs, said Webroot. 

"We want to make sure that users understand Vista's limitations, and that default malware blocking and antivirus programs may not fully protect them," said Webroot senior vice president of engineering Gerhard Eschelbeck.

The company said that Windows Defender was outperformed in the test by numerous third-party vendors, and that users should stick to companies that specialise in security software to provide optimal protection.

"We feel strongly that, in order to provide the best protection for internet users, security must be your top and only priority," said Eschelbeck.

Windows Vista has consistently been the target of criticism from the security community since Microsoft revealed that the operating system's kernel would be sealed off from developers.

Microsoft claims that its Patchguard feature will reduce the effectiveness of malware by cutting off access to the core of the operating system.

Security researchers and developers, however, contend that hackers will inevitably penetrate Patchguard's defences and attack the Vista kernel, leaving security software unable to stop the exploit.

Microsoft maintained that Vista's built-in security features are not intended to be the only line of defence.

A spokesman told vnunet.com: "While Windows Vista was engineered to be the most secure version of Windows yet it is important to note that no operating system is 100 per cent secure." 

The company recommends that all users install additional antivirus and security software.