Windows Vista is more susceptible to malware than the eight year-old Windows 2000, according to security vendor PC Tools Software.

Statistics garnered from users of PC Tools' ThreatFire security service showed that Vista let through 639 threats per 1,000 computers, compared with 586 for Windows 2000, 478 for Windows 2003, and 1,021 for Windows XP.

Working on an infection rate of 639 per 1,000 PCs means that nearly 64 per cent of Vista-based PCs have already been compromised, the company claims.

PC Tools, which makes its money from vulnerabilities in Microsoft's software, predicted further increases in infection rates.

"If Microsoft's forecasts for the operating system are correct, and Vista's market share increases significantly, we could expect infection rates to increase on Vista," said Simon Clausen, chief executive at PC Tools.

Clausen added that recent research conducted from over 1.4 million computers within the ThreatFire community had shown that Windows Vista was only 37 per cent more secure than Windows XP.

A Microsoft spokesperson said in response that tests carried out by the company on more than 400 million machines contradicted PC Tools' findings.

"We appreciate independent studies and encourage researchers to help us make our products more secure, but this is a study by a vendor of anti-malware products," said the spokesperson said in an emailed statement.

"ThreatFire vulnerability comparison numbers certainly do not reflect our vulnerability findings from the Malicious Software Removal Tool which ran on over 400 million machines in December 2007."