Microsoft has touted Vista as a more secure version of Windows, but on the day of Vista's official launch, a security company identified malware already in circulation that can infect computers running the OS.
Sophos identified three viruses typically spread through email that can infect Vista customers who use a third party Web email client.
While Vista's email client stopped Stratio-Zip, Netsky-D and MyDoom-O, the malware slipped past Vista's defenses when users received infected messages through a Web-based email service, Sophos said.
Stratio-Zip topped Sophos' list of malware affecting computer users in the month of November, accounting for 33.3 per cent of malware in circulation. Combined, the three viruses that can affect Vista users make up 39.7 per cent of all malware in circulation during the month, Sophos said.
However, even if the malware Sophos identified slips through in an email, customers wouldn't necessarily be affected, another researcher said.
Additional Vista security mechanisms should protect users, chief research officer at F-Secure, Mikko Hyppaunen, said. If a customer opened an infected malware file, Vista would warn and question the user before allowing the malware to wreak havoc.
"These particular examples of malware probably wouldn't still be able to successfully infect the machine unless the user specifically allows it," he wrote in an email exchange.
Sophos applauded the security improvements in Vista, claiming that the variety of popular third party applications used by consumers inevitably would open doors to hackers.
Other antivirus companies haven't been so kind. McAfee has been highly critical of changes in the operating system that it claims will make Vista less secure than previous versions of Windows.
Symantec said it has discovered vulnerabilities in Vista's networking software which made it less stable than Windows XP.
Sophos found that overall, the proportion of infected email remained low in November at 0.28 per cent, but identified a record number of new threats, 7612, during the month.